Tail Lobe Magnetic Field Research Articles

Magnetotail magnetic flux monitoring based on simultaneous solar wind and magnetotail observations

AbstractThe magnetotail magnetic flux (MTF) is an important global variable to describe the magnetospheric state and dynamics. Existing methods of MTF estimation on the basis of the polar cap area, inferred from observations of global auroras and field‐aligned currents, do not allow benchmarking due to the absence of a gauge for comparison; besides, they rarely allow a systematic nearly real time MTF monitoring. We describe three modifications (F0, F1, and F2) of the method to calculate the MTF, based on simultaneous spacecraft observations in the magnetotail and in the solar wind, suitable for real‐time MTF monitoring. The MTF dependence on the solar wind parameters and the observed tail lobe magnetic field is derived from the pressure balance conditions. An essential part of this study is the calibration of our approximate method against global 3‐D MHD simulations and the empirical T14 magnetospheric field model. The calibration procedure provides all variables required to evaluate F0, F1, and F2 quantities and, at the same time, computes the reference MTF value through any tail cross section. It allowed us to extend the method to be used in the near tail, investigate its errors, and define the applicability domain. The method was applied to Cluster and THEMIS measurements and compared with methods of polar cap area calculation based on IMAGE and AMPERE observations. We also discuss possible applications and some recent results based on the proposed method.

Verification of the GUMICS‐4 global MHD code using empirical relationships

Global magnetohydrodynamic (MHD) modeling is a powerful tool in space physics research. There are several advanced and still developing global MHD codes that are widely used to simulate plasma processes in solar wind magnetosphere‐ionosphere system. The verification of global simulation codes is an important but a difficult problem. We present an approach for systematic and quantitative testing of code performance based on statistical empirical dependencies of the key magnetospheric parameters obtained from observations. We demonstrate the applicability of the method by testing the Grand Unified Magnetosphere Ionosphere Coupling simulation (GUMICS‐4) global MHD model. A large set of nearly stationary solutions (162 runs altogether) with different stationary interplanetary magnetic field (IMF) and solar wind inputs were generated for different dipole tilts and levels of solar EUV radiation. As key parameters, we use the large‐scale characteristics of the magnetosphere, including the magnetopause size and shape, geometry of the tail neutral sheet, magnetotail plasma pressure, tail lobe magnetic field, and cross‐polar cap electric potential. We found that the GUMICS‐4 stationary solutions generally fit the statistical relations, however, with some discrepancies. Particularly, position of the subsolar magnetopause, neutral sheet shape and position, and the plasma sheet pressure during northward IMF agree well with statistical models. At the same time, the size of the tail magnetopause and the lobe magnetic field magnitude appear to be systematically lower compared to their empirical values. Furthermore, the ionospheric potential is smaller in magnitude compared to empirical relations. These results provide an important starting point in the further development of the GUMICS simulation.

On the retreat of near-Earth neutral line during substorm expansion phase: a THEMIS case study during the 9 January 2008 substorm

Abstract. The location of magnetic reconnection in the mid-tail during a substorm was studied in many researches. Here we present multi-point THEMIS observations of a reconnection event in the near-Earth magnetotail during substorm. In this event, THEMIS probes stayed in the near-Earth and mid-tail region aligning along the magnetotail. This allows reconnection evolution to be probed simultaneously from about −10 RE to −23 RE down tail. The Hall current related electron streams were observed at the same time by two probes far away from the reconnection site. Before near-Earth reconnection involved the tail lobe magnetic field, the reconnection site was restricted in earthward −23 RE. When reconnection involved into the tail lobe region, the reconnection site started to retreat gradually.

Cluster observations of sudden impulses in the magnetotail caused by interplanetary shocks and pressure increases

Abstract. Sudden impulses (SI) in the tail lobe magnetic field associated with solar wind pressure enhancements are investigated using measurements from Cluster. The magnetic field components during the SIs change in a manner consistent with the assumption that an antisunward moving lateral pressure enhancement compresses the magnetotail axisymmetrically. We found that the maximum variance SI unit vectors were nearly aligned with the associated interplanetary shock normals. For two of the tail lobe SI events during which Cluster was located close to the tail boundary, Cluster observed the inward moving magnetopause. During both events, the spacecraft location changed from the lobe to the magnetospheric boundary layer. During the event on 6 November 2001 the magnetopause was compressed past Cluster. We applied the 2-D Cartesian model developed by collier98 in which a vacuum uniform tail lobe magnetic field is compressed by a step-like pressure increase. The model underestimates the compression of the magnetic field, but it fits the magnetic field maximum variance component well. For events for which we could determine the shock normal orientation, the differences between the observed and calculated shock propagation times from the location of WIND/Geotail to the location of Cluster were small. The propagation speeds of the SIs between the Cluster spacecraft were comparable to the solar wind speed. Our results suggest that the observed tail lobe SIs are due to lateral increases in solar wind dynamic pressure outside the magnetotail boundary.

Magnetotail behavior during storm time “sawtooth injections”

A series of magnetic storms occurred during 16–21 April 2002 with multiple local minima in the Dst index. During this storm period, Geotail traversed the magnetotail from the premidnight tail lobe in the midtail region to the postmidnight plasma sheet in the near‐Earth region. Simultaneous observations of energetic particles at the geostationary altitude showed the feature known as “sawtooth injections” remarkably well from 18 April to 19 April. This conjunction allows us to examine the magnetotail behavior in the tail lobe and the plasma sheet associated with sawtooth injections. In the tail lobe, close but not perfect time coincidence is found between injection onset and start of southward dipping of the magnetic field vector in the tail lobe. Long‐duration (1–2 hours) southward dipping in the tail lobe magnetic field appears to be a global behavior of the tail lobe in association with sawtooth injections. In the plasma sheet, intermittent occurrences of plasma flow reversal from sunward to tailward and of magnetic field from northward to southward are found. The duration of southward magnetic field is typically short (1–5 min) within the plasma sheet. There are some sawtooth injections that are not accompanied by any significant changes in plasma sheet parameters (plasma flows, magnetic and electric fluctuation levels), suggesting the plasma sheet behavior to be localized even in association with sawtooth injections that are global in nature. There was no indication of the magnetotail being driven in a relatively “steady state” in association with sawtooth injections during this sequence of storms.

Average characteristics of the midtail plasma sheet in different dynamic regimes of the magnetosphere

Abstract. We study average characteristics of plasma sheet convection in the middle tail during different magnetospheric states (Steady Magnetospheric Convection, SMC, and substorms) using simultaneous magnetotail (Geotail, 15-35 RE downtail) and solar wind (Wind spacecraft) observations during 3.5 years. (1) A large data set allowed us to obtain the average values of the plasma sheet magnetic flux transfer rate (Ey and directly compare it with the dayside transfer rate (Emod for different magnetospheric states. The results confirm the magnetic flux imbalance model suggested by Russell and McPherron (1973), namely: during SMC periods the day-to-night flux transport rate equals the global Earthward plasma sheet convection; during the substorm growth phase the plasma sheet convection is suppressed on the average by 40%, whereas during the substorm expansion phase it twice exceeds the day-to-night global flux transfer rate. (2) Different types of substorms were revealed. About 1/3 of all substorms considered displayed very weak growth in the tail lobe magnetic field before the onset. For these events the plasma sheet transport was found to be in a balance with the day-to-night flux transfer, as in the SMC events. However, the lobe magnetic field value in these cases was as large as that in the substorms with a classic growth phase just before the onset (both values exceed the average level of the lobe field during the SMC). Also, in both groups similar configurational changes (magnetic field stretching and plasma sheet thinning) were observed before the substorm onset. (3) Superimposed epoch analysis showed that the plasma sheet during the late substorm recovery phase has the characteristics similar to those found during SMC events, the SMC could be a natural magnetospheric state following the substorm.

Tail plasma sheet models derived from Geotail particle data

Simple analytical models have been derived for the first time, describing the 2‐D distribution (along and across the Earth's magnetotail) of the central plasma sheet (CPS) ion temperature, density, and pressure, as functions of the incoming solar wind and interplanetary magnetic field (IMF) parameters, at distances between 10 and 50 RE. The models are based on a large set of data of the Low‐Energy Particle (LEP) and Magnetic Field (MGF) instruments, taken by Geotail spacecraft between 1994 and 1998, comprising 7234 1‐min average values of the CPS temperature and density. Concurrent solar wind and IMF data were provided by the Wind and IMP 8 spacecraft. The accuracy of the models was gauged by the correlation coefficient (c.c.) R between the observed and predicted values of a parameter. The CPS ion density N is controlled mostly by the solar wind proton density and by the northward component of the IMF. Being the least stable characteristic of the CPS, it yielded the lowest c.c. RN = 0.57. The CPS temperature T, controlled mainly by the solar wind speed V and the IMF Bz, gave a higher c.c. RT = 0.71. The CPS ion pressure P was best controlled by the solar wind ram pressure Psw and by an IMF‐related parameter F = B⟂, where B⟂ is the perpendicular component of the IMF and θ is its clock angle. In a striking contrast with N and T, the model pressure P revealed a very high c.c. with the data, RP = 0.95, an apparent consequence of the force balance between the CPS and the tail lobe magnetic field. No significant dawn‐dusk asymmetry of the CPS was found beyond the distance 10 RE, in line with the observed symmetry of the tail lobe magnetic field. The plasma density N is lowest at midnight and increases toward the tail's flanks. Larger (smaller) solar wind ion densities and northward (southward) IMF Bz result in larger (smaller) N in the CPS. In contrast to the density N, the temperature T peaks at the midnight meridian and falls off toward the dawn/dusk flanks. Faster (slower) solar wind flow and southward (northward) IMF Bz result in a hotter (cooler) CPS. The CPS ion pressure P is essentially a function of only XGSM in the midtail (20–50 RE); at closer distances the isobars gradually bend to approximately follow the contours of constant geomagnetic field strength. For northward IMF conditions combined with a slow solar wind, the isobars remain quasi‐circular up to larger distances, reflecting a weaker tail current and, hence, more dipole‐like magnetic field.

Energy Flux in the Earth’s Magnetosphere: Storm-Substorm Relationship

Three ways of the energy transfer in the Earth’s magnetosphere are studied. The solar wind MHD generator is an unique energy source for all magnetospheric processes. Field-aligned currents directly transport the energy and momentum of the solar wind plasma to the Earth’s ionosphere. The magnetospheric lobe and plasma sheet convection generated by the solar wind is another magnetospheric energy source. Plasma sheet particles and cold ionospheric polar wind ions are accelerated by convection electric field. After energetic particle precipitation into the upper atmosphere the solar wind energy is transferred into the ionosphere and atmosphere. This way of the energy transfer can include the tail lobe magnetic field energy storage connected with the increase of the tail current during the southward IMF. After that the magnetospheric substorm occurs. The model calculations of the magnetospheric energy give possibility to determine the ground state of the magnetosphere, and to calculate relative contributions of the tail current, ring current and field-aligned currents to the magnetospheric energy. The magnetospheric substorms and storms manifest that the permanent solar wind energy transfer ways are not enough for the covering of the solar wind energy input into the magnetosphere. Nonlinear explosive processes are necessary for the energy transmission into the ionosphere and atmosphere. For understanding a relation between substorm and storm it is necessary to take into account that they are the concurrent energy transferring ways.

Solar wind control of the tail lobe magnetic field as deduced from Geotail, AMPTE/IRM, and ISEE 2 data

A statistical study was made of the tail lobe field response to the dynamical pressure of the incoming solar wind and to the interplanetary magnetic field, as well as its relation to the concurrent level of the Dst field. The study covers a wide range of distances between 10 and 60RE and is based on a large set of tail lobe magnetic field data, compiled from three sources: (1) Geotail magnetometer and low‐energy plasma instrument data for 1993–1997, (2) AMPTE/IRM magnetometer and plasma instrument data for 1985–1986, and (3) ISEE 2 magnetometer and fast plasma experiment data for 1978–1980. The tailward variation of the tail lobe field and of its response to the solar wind and interplanetary magnetic field (IMF) conditions was studied using a regression relationship, including various combinations of the interplanetary quantities, measured by IMP 8 and Wind spacecraft. An investigation of the role of the time lag effects was made by tagging each lobe field measurement by a “trail” of 12 consecutive 5‐min average values of the solar wind parameters and finding best fit distributions of the lagged response amplitudes in the solar wind and IMF‐related regression terms. The goal of the work is to find a set of input variables providing the highest correlation between the observed and predicted magnitude of the tail lobe field, so that their combination could be used for parameterizing the strength of the cross‐tail current in the data‐based magnetospheric models.

Estimating the long‐term variations of the magnetotail pressure

Semiannual values of the dynamic pressure and the static pressure in the solar wind are calculated for 1965‐1988. Using the statistical relationship between the tail lobe magnetic field and solar wind parameters, variations of the magnetic field pressure in the magnetic lobe at X=−20 RE are predicted. This prediction is compared with various satellite observations of the total pressure, although systematic observations were not available on a continuous basis. It is predicted that the semiannual value of the magnetotail total pressure exhibits the variation with a period of ≈11 years in close conjunction with the solar cycles. The magnetotail total pressure tends to be minimum at the maximum epoch of the sunspot number.

Plasma sheet convection and the stability of the magnetotail

Particle simulations are used to investigate the effects of plasma sheet convection into regions of increasing tail lobe magnetic field strength on the stability of a magnetotail equilibrium. The self‐consistent treatment of convection first drives Bz to zero on axis in the region of the strongest lobe field Bz and then causes the equilibrium to break due to the rapid growth of tearing modes driven by the induced temperature anisotropy. The net result is the evolution of the inward‐convecting plasma sheet into a slowly moving or stagnant plasmoid. The time scale for this process is much more rapid than that associated with particle drift losses across the tail. These results support the suggestion that steady convection within the plasma sheet may not be possible.

IMP‐8 observations of traveling compression regions: New evidence for near‐Earth plasmoids and neutral lines

An examination of IMP‐8 tail lobe magnetic field measurements has been conducted to determine whether the traveling compression region (TCR) phenomena detected by ISEE‐3 in the distant geotail, and believed to be caused by tailward moving plasmoids, are present closer to the earth. The study produced 16 examples of TCRs at distances of X = −31 to −37 RE. For two events considered in detail TCRs were observed in close association with substorm growth phase signatures in the lobes. The lengths of these TCRs are estimated to be 8–12 RE. It is our conclusion that the IMP‐8 TCR observations provide new evidence that small plasmoids and, hence, multiple reconnection neutral lines can exist earthward of X = −35 RE.

Low‐altitude convection, precipitation, and current patterns in the baseline magnetosphere

Three states of the magnetosphere may be identified from polar auroral and magnetic activity: the active auroral oval state, the active polar cap state, and the quiet state. The quiet time magnetosphere occurs when the energy transfer from the solar wind has been maintained at a minimum level for several hours. The conditions for minimal energy transfer occur for low solar wind velocity and for small IMF B and Bz. The quiet magnetosphere is characterized by minimal, but nonzero, cross‐cap electric field potentials (6–20 kV) and, therefore, low earthward convection velocities in the plasma sheet. The auroral oval shrinks to a circle of radius 20° geomagnetic latitude (MLAT) offset, as in active times, toward midnight. Diffuse auroral precipitation is continuous throughout these periods but has low average energy and number flux. Discrete arcs are weak (subvisual) and multiple, often extending to extremely high latitudes, particularly on the dayside of the cap. The cusp lies between 77° and 84° MLAT at noon. The total particle energy flux into both hemispheres is ∼ 10 GW. Region 1 and region 2 field‐aligned current systems frequently are not observed during extended periods of magnetic quiet. However, small‐scale current systems, which are associated with the weak discrete arcs, are still seen. Cusp currents in the sunlit hemisphere remain strong. Of particular interest in modeling the quiet magnetosphere are the following topics: the entry and transport of cusp particle populations, stable convection in the plasma sheet, and the closure of tail lobe magnetic field lines.

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.

An empirical relationship between Kp and the tail lobe magnetic field

In this paper a useful empirical relationship is established between the readily available magnetic activity index, Kp, and the tail lobe magnetic field strength. A simple (dipole plus tail field plus ring current) magnetic field model is combined with the results of Gussenhoven et al. (1981) that relate the location of the equatorward boundary of auroral oval with Kp. The results are tested by taking over 500 samples of Kp and converting them to the corresponding lobe field strength values which are then plotted as a frequency distribution. This distribution is compared with a similar distribution of over 700 hours of IMP 4 tail lobe magnetic field measurements taken during the same time period by Fairfield and Ness (1970). The agreement between the two distributions is excellent only when a model ring current is included. This implies that the ring current makes a significant contribution to the location of the equatorward boundary of the auroral oval (Siscoe, 1979), particularly during periods of high Kp. Comparisons are made between the present model and the zero‐energy Alfven layer calculations that have appeared in the literature.

The statistical magnetic signature of magnetospheric substorms

The characteristic magnetic signatures of magnetospheric substorms both on the ground and in space have been determined from the analysis of ∼1800 substorm events. The timing and properties of these events were objectively determined according to explicit mathematical criteria by a computer pattern-recognition program. This program processed daily magnetograms from a mid-latitude network of geomagnetic observatories. Ground data analyzed, using onsets determined in this manner, included the AE indices and individual magnetograms at different local times in the auroral zone and at midlatitudes. Superposed epoch averages of these data confirm the local time magnetic substorm signatures, determined in earlier studies of fewer events, and demonstrate the validity of the computerized onset determination procedure. Superposed epoch averages of the interplanetary magnetic field (IMF) associated with the onsets demonstrates both a distinct southward component prior to the onsets and a dependence of the substorm amplitude on the integrated preceding southward IMF flux. Superposed epoch averages of the tail lobe magnetic field magnitude and vector components demonstrates field magnitude changes and rotations in association with the substorm onsets. These lobe field changes are consistent with the growth-phase model of substorm activity and with variations in the magnetopause flaring angle.