I. I. Alexeev

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.

Modeling of geomagnetic field during magnetic storms and comparison with observations

This paper discusses: (a) development of the dynamic paraboloid magnetospheric (eld model, (b) application of this model for the evaluation of a variety of magnetospheric current systems and their contribution to the ground magnetic (eld variations during magnetic storms, (c) investigation of auroral electrojet dynamics and behavior of plasma precipitation boundaries, and (d) usage of the paraboloid magnetospheric (eld model for revealing relationships between geomagnetic phenomena at low altitudes and the large-scale magnetospheric plasma domains. The model’s input parameters are determined by the solar wind plasma velocity and density, the IMF strength and direction, the tail lobe magnetic 6ux F∞, and the total energy of ring current particles. The auroral particle precipitation boundaries are determined from, the DMSP particle observations; these boundaries are used to calculate the value of F∞. The in6uence of the (eld-aligned tail, and ring currents on the magnetospheric (eld structure is studied. It is found that the polar cap area is strongly controlled by the tail current. The paraboloid magnetospheric (eld model is utilized for the mapping of the auroral electrojet centerlines and boundaries into the magnetosphere. Analysis of the magnetic (eld variations during magnetic storms shows that the contributions of the ring current, tail current, and the magnetopause currents to the Dst variation are approximately equal. c

Concerning the location of magnetopause merging as a function of the magnetopause current strength

We start from an assumption that merging occurs in regions of the magnetopause where current strengths are greater than some threshold value which corresponds to the total jump in the field across the magnetopause greater than 50 nT. Because time and cost constraints preclude running numerical simulations for a wide variety of interplanetary magnetic field (IMF) orientations to determine these locations, we adopt an analytical model based on previously derived formulations for magnetospheric and magnetosheath magnetic fields. The magnetospheric magnetic field is confined within a paraboloid. The magnetosheath magnetic field is derived from that in the solar wind and lies between the magnetopause and a paraboloid bow shock. We allow a slight diffusion of the magnetosheath magnetic field into the magnetosphere. The results of the model show that during periods of due southward IMF orientation, merging occurs (as expected) in a wide region centered on the subsolar magnetopause. During periods of northward IMF, connection continues near the subsolar point but also poleward of the cusps. Magnetic energy is only released to the plasma in the latter regions. During periods of strongly northward IMF (By = 0), reconnection ceases on the subsolar magnetopause but continues poleward of the cusp. If the IMF points northward but By is nonzero, reconnection continues near the subsolar point and poleward of the cusps. During periods of sunward IMF orientation, merging nearly ceases on the northern hemisphere (except in the vicinity of the subsolar point) but continues outside the southern lobes. Dawnward and duskward IMF orientations produce tilted patches of enhanced current densities in the subsolar region. We compare the results of our model with previous predictions of the “component” and “antiparallel” merging models.

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

Energetics of the magnetosphere during the magnetic storm

Abstract The most detailed studies of the energetic budget of the magnetosphere during the magnetic storms were done on the basis of the paraboloid model using the November 23–27, 1986 and May 6–8, 1988 magnetic storms. Calculations have shown that the energy injected in the course of the magnetic storms into the inner magnetosphere and ionosphere of both hemispheres amounts to ∼ 0.9 –2.2% of the solar wind kinetic energy on the magnetospheric cross section. The total energy injected into the magnetosphere from a distance of 60RE in the tail down to the ionosphere is ∼4.0–7.5% of the solar wind kinetic energy during the main phase of the two magnetic storms. The injected energy into the tail ETL is 1.03–1.18 of the total energy input into the inner magnetosphere and ionosphere of both hemispheres at the main phase of the two storms. The decay parameter for the energy stored in the magnetospheric tail is ∼5 h . The total energy dissipated in the ionosphere of both hemispheres, in the inner magnetosphere and in the tail during the two storms, is 1.85×1017 and 3.24×10 17 J , respectively. The total energy input into the magnetosphere is calculated to be 1.77×1017 and 3.16×10 17 J . The discrepancies of 0.08×1017 and 0.10×10 17 J amount to 4.3% and 3.1% of the total energy input and characterize the accuracy of the magnetospheric energy budget calculation. In the magnetotail the balance between the injected and dissipated energy of ∼1.09×10 17 J for one storm and ∼1.7×10 17 J for the other is preserved as well. We conclude that one-half of the energy which is injected into the magnetosphere from the solar wind during the storms enters the magnetotail and dissipates there. The coupling parameter ePA is widely considered to be a measure of the energy dissipation in the inner magnetosphere. The dissipation energy in the inner magnetosphere UT=UJ+UA+UDR is defined as the sum of the contributions of the Joule dissipation UJ, the energy of auroral particle precipitation UA, and the energy injection into the ring current UDR. In this paper, we find that ePA in the two storms investigated is substantially different from UT. The energy injected into the ring current region at the main phase of the storm amounts to ∼10 16 J . It is nearly 3 or 4 times smaller than the energy input into the magnetosphere via the field-aligned currents or the energy dissipated in the ionosphere by Joule dissipation. The energy injected into the magnetosphere is transferred mainly into processes different from the ring current generation. During the development of intensive auroral electrojets, the energy dissipation in the magnetotail and the increase in the energy of the tail current system occur simultaneously. The energy dissipation in the inner magnetosphere and ionosphere US occurs not only at the expense of energy previously stored in the magnetotail, but rather at the expense of energy that is injected into the near-Earth tail. This energy transfer from the solar wind into the magnetotail and the energy dissipation in the ionosphere increased simultaneously. Thus, during disturbances in the magnetosphere, simultaneously loading–unloading and directly driven processes occur. Loading- and unloading processes manifest themselves both in the storage of the solar wind energy in the magnetotail and the ring current, and subsequent dissipation. The directly driven processes become manifest in the direct dissipation of the energy which enters into the ionosphere through large-scale field-aligned current systems.