E. S. Belenkaya

Magnetic storms and magnetotail currents

The magnetospheric magnetic field is highly time-dependent and may have explosive changes (magnetospheric substorms and geomagnetic storms) accompanied by significant energy input into the magnetosphere. However, the existing stationary magnetospheric models can not simulate the magnetosphere for disturbed conditions associated with the most interesting magnetospheric physics events (intensive auroras, particle injection in the inner magnetosphere, and precipitations at the high latitudes, etc.). We propose a method for constructing a nonstationary model of the magnetospheric magnetic field, which enables us to describe the magnetosphere during the disturbances. The dynamic changes of the magnetosphere will be represented as a sequence of quasistationary states. The relative contributions to the Dst index by various sources of magnetospheric magnetic field are considered using a dynamic model of the Earths magnetosphere. The calculated magnetic field is obtained by using the solar wind and geomagnetic activity empirical data of the magnetic storm of March 23–24, 1969 and the magnetic disturbance of July 24–26, 1986. The main emphasis is on the current system of the magnetospheric tail, the variations of which enable a description of the fast changes of Dst.

Magnetic storms in October 2003

Preliminary results of an analysis of satellite and ground-based measurements during extremely strong magnetic storms at the end of October 2003 are presented, including some numerical modeling. The geosynchronous satellites Ekspress-A2and Ekspress-A3, and the low-altitude polar satellites Coronas-F and Meteor-3M carried out measurements of charged particles (electrons, protons, and ions) of solar and magnetospheric origin in a wide energy range. Disturbances of the geomagnetic field caused by extremely high activity on the Sun were studied at more than twenty magnetic stations from Lovozero (Murmansk region) to Tixie (Sakha-Yakutia). Unique data on the dynamics of the ionosphere, riometric absorption, geomagnetic pulsations, and aurora observations at mid-latitudes are obtained.

Field‐aligned currents in Saturn's northern nightside magnetosphere: Evidence for interhemispheric current flow associated with planetary period oscillations

We investigate the magnetic perturbations associated with field-aligned currents observed on 34 Cassini passes over the premidnight northern auroral region during 2008. These are found to be significantly modulated not only by the northern planetary-period oscillation (PPO) system, similar to the southern currents by the southern PPO system found previously, but also by the southern PPO system as well, thus providing the first clear evidence of PPO-related interhemispheric current flow. The principal field-aligned currents of the two PPO systems are found to be co-located in northern ionospheric colatitude, together with the currents of the PPO-independent (subcorotation) system, located between the vicinity of the open-closed field boundary and field lines mapping to ~9 Saturn radius (Rs) in the equatorial plane. All three systems are of comparable magnitude, ~3 MA in each PPO half-cycle. Smaller PPO-related field-aligned currents of opposite polarity also flow in the interior region, mapping between ~6 and ~9 Rs in the equatorial plane, carrying a current of ~ ±2 MA per half-cycle, which significantly reduce the oscillation amplitudes in the interior region. Within this interior region the amplitudes of the northern and southern oscillations are found to fall continuously with distance along the field lines from the corresponding hemisphere, thus showing the presence of cross-field currents, with the southern oscillations being dominant in the south, and modestly lower in amplitude than the northern oscillations in the north. As in previous studies, no oscillations related to the opposite hemisphere are found on open field lines in either hemisphere.

Dynamic model of the magnetosphere: Case study for January 9–12, 1997

The dynamics of the magnetospheric current systems are studied in the course of the specific magnetospheric disturbance on January 9–12, 1997, caused by the interaction of the Earths magnetosphere with a dense solar wind plasma cloud. To estimate the contribution of the different sources of the magnetospheric magnetic field to the disturbance ground measured, a dynamic paraboloid model of the magnetosphere is used. The model input parameters are defined by the solar wind density and velocity, by the strength and direction of the interplanetary magnetic field, and by the auroral AL index. The total energy of the ring current particles is calculated from the energy balance equation, where the injection function is determined by the value of the solar wind electric field. New analytical relations describing the dynamics of the different magnetospheric magnetic field sources dependent on the model input parameters are obtained. The analysis of the magnetic disturbances during the January 9–12, 1997, event shows that in the course of the main phase of the magnetic storm the contribution of the ring current, the currents on the magnetopause, and the currents in the magnetotail are approximately equal to each other by an order of magnitude. Nevertheless, in some periods one of the current systems becomes dominant. For example, an intense Dst positive enhancement (up to +50 nT) in the course of the magnetic storm recovery phase in the first hours on January 11, 1997, is associated with a significant increase of the currents on the magnetopause, while the ring current and the magnetotail current remain at a quiet level. A comparison of the calculated Dst variation with measurements indicates good agreement. The root mean square deviation is ∼ 8.7 nT in the course of the storm.

Special features of the September 24–27, 1998 storm during high solar wind dynamic pressure and northward interplanetary magnetic field

The geomagnetic storm on September 24 – 27, 1998, was initiated by a sudden compression of the magnetosphere in response to the solar wind dynamic pressure pulse. Simultaneous with the pressure increase, the interplanetary magnetic field (IMF) became strongly northward. Several unexpected magnetospheric responses to this sudden impulse were observed. First, for ∼ 30 min following the sudden impulse, the entire auroral oval became active and thick, while the polar cap area decreased to less than 1/2 of its original size. The second unusual observation associated with the sudden impulse is the global magnetic perturbation measured by low-latitude magnetic stations. The field shows an asymmetric increase in the axial component (parallel to the dipole axis) with the strongest enhancement measured on the night side and at local magnetic noon the perturbation is small or slightly negative. This is very unusual since sudden compressions are generally measured by low-latitude stations to have the largest enhancement of the field on the dayside. The main phase of the geomagnetic storm begins in the second hour on September 25, 1998, following the southward turning of the IMF. The auroral oval becomes thin and moves equatorward increasing the polar cap area by more than a factor of 3. The theoretical analysis presented in this paper suggests that the response to the sudden impulse is an electrodynamic effect produced by a “transition” current system in response to the northward turning of the IMF.

A model of region 1 field‐aligned currents dependent on ionospheric conductivity and solar wind parameters

Using a paraboloid model of the magnetosphere, we compute the potential drop across open field lines as a function of the interplanetary magnetic field strength and direction as well as of solar wind velocity and density. The collisionless conductivity of the magnetosheath plasma near the magnetopause determines the efficiency of the solar wind electric field penetration into the magnetosphere. This reconnection efficiency is generally about 0.1. Thus about one tenth of the total potential produced over the magnetospheric cross section penetrates the polar cap. Knowing the potential drop across the polar cap allows us to determine the strength of the region 1 field-aligned currents as a function of ionospheric conductivity. We compute the magnetic field disturbance produced by the region 1 field-aligned currents by using a simple current loop in which the region 1 field-aligned currents close through ionospheric and magnetopause currents. The region 1 field-aligned currents decrease the magnetic field strength on the dayside of the magnetosphere, moving the cusp to lower latitudes. This corresponds to a sunward displacement of the polar cap and polar oval. The displacement of the polar oval is 8° on the dayside and about 3° on the nightside when the total strength of the region l Birkeland currents is 5 MA.

Saturn's dayside ultraviolet auroras: Evidence for morphological dependence on the direction of the upstream interplanetary magnetic field.

We examine a unique data set from seven Hubble Space Telescope (HST) “visits” that imaged Saturns northern dayside ultraviolet emissions exhibiting usual circumpolar “auroral oval” morphologies, during which Cassini measured the interplanetary magnetic field (IMF) upstream of Saturns bow shock over intervals of several hours. The auroras generally consist of a dawn arc extending toward noon centered near ∼15° colatitude, together with intermittent patchy forms at ∼10° colatitude and poleward thereof, located between noon and dusk. The dawn arc is a persistent feature, but exhibits variations in position, width, and intensity, which have no clear relationship with the concurrent IMF. However, the patchy postnoon auroras are found to relate to the (suitably lagged and averaged) IMF Bz, being present during all four visits with positive Bz and absent during all three visits with negative Bz. The most continuous such forms occur in the case of strongest positive Bz. These results suggest that the postnoon forms are associated with reconnection and open flux production at Saturns magnetopause, related to the similarly interpreted bifurcated auroral arc structures previously observed in this local time sector in Cassini Ultraviolet Imaging Spectrograph data, whose details remain unresolved in these HST images. One of the intervals with negative IMF Bz however exhibits a prenoon patch of very high latitude emission extending poleward of the dawn arc to the magnetic/spin pole, suggestive of the occurrence of lobe reconnection. Overall, these data provide evidence of significant IMF dependence in the morphology of Saturns dayside auroras. Key Points We examine seven cases of joint HST Saturn auroral images and Cassini IMF data The persistent but variable dawn arc shows no obvious IMF dependence Patchy postnoon auroras are present for northward IMF but not for southward IMF

Electric fields and field-aligned current generation in the magnetosphere

The authors present a calculation of the electric potential, field-aligned currents, and plasma convection caused by the penetration of the solar wind electric field into the magnetosphere. Ohms law and the continuity equation of ionospheric currents are used. It is shown that the large-scale convection system is reversed in the plasma sheet flanks. In this region the plasma flow is antisunward earthward of the neutral line and sunward tailward of it. The interplanetary magnetic field (IMF) B[sub z] dependences on the dimension of the magnetopause [open quotes]windows[close quotes] which are intersected by open field lines, on the potential drop across the polar cap, and on the distance to the neutral line are determined. Because of the IMF effect and the effect of seasonal or daily variations of the geomagnetic field which violate its symmetry relative to the equatorial plane, there may arise a potential drop along field lines which causes field-aligned currents. The values and directions of these currents, the field-aligned potential drop, and a self-consistent solution for the potential at the ionosphere level for high field-aligned conductivity have been determined. 41 refs., 7 figs.

High-latitude ionospheric convection patterns dependent on the variable imf orientation

Abstract The magnetospheric magnetic field configuration, created by merging, depends strongly on the interplanetary magnetic field (IMF) orientation. There are different types of reconnection: two-dimensional reconnection at a merging line lying within the magnetopause current layer and⧹or within the magnetotail, and three-dimensional reconnection at the magnetic field neutral point located inside the magnetosphere, earthward of the magnetopause. All orientations of the IMF are divided into four sectors. For the first sector reconnection of the Dungey-type occurs; for the second one three-dimensional reconnection takes place at two neutral points inside the magnetosphere; for the third—one neutral point exists inside the magnetosphere, near the northern cusp, and the other is within the southern magnetopause current layer. For the fourth sector one neutral point is located earthward of the southern magnetopause, and the other—within the northern magnetopause current sheet. The high-latitude northern and southern ionospheric convection patterns depend upon the solar wind-magnetosphere coupling type and are different for each sector of IMF-orientation.

Investigation of scaling properties of a thin current sheet by means of particle trajectories study

A thin current sheet (TCS), with the width of an order of thermal proton gyroradius, appears a fundamental physical object which plays an important role in structuring of major magnetospheric current systems (magnetotail, magnetodisk, etc.). The TCSs are nowadays under extensive study by means of space missions and theoretical models. We consider a simple model of the TCS separating two half-spaces occupied by a homogenous magnetic field of opposite sign tangential to the TCS; a small normal component of the magnetic field is prescribed. An analytical solution for the electric current and plasma density in the close vicinity of the TCS has been obtained and compared with numerical simulation. The number density and the electric current profiles have two maxima each. The characteristic spatial scale zS of the maxima location was investigated as a function of initial pitch angle of an incoming charge particle. The effect of the thermal dispersion of the incoming proton beam have been taken into consideration.