Long-term evolution of magnetospheric current systems during storms

Abstract

Abstract. We present a method to model the storm-time magnetospheric magnetic field using representations of the magnetic field arising from the various magnetospheric current systems. We incorporate the effects of magnetotail changes during substorms by introducing an additional localized thin current sheet into the Tsyganenko T89 model. To represent the storm-time ring current the T89 ring current is replaced by a bean-shaped current system, which has a cross section that is close to the observed distribution of trapped particles in the inner magnetosphere and has an eastward flowing inner and westward flowing outer components. In addition to the symmetric ring current, an asymmetric partial ring current is taken into account with closing Region 2 sense field-aligned currents. Magnetopause currents are varied in accordance with solar wind dynamic pressure variations. Three moderate geomagnetic storms when D st reached about −150 nT and one big storm with D st about −250 nT are modelled. The model free parameters are specified for each time step separately using observations from GOES 8 and 9, Polar, Interball and Geotail satellites and D st measurements. The model gives a high time-resolution field representation of the large-scale magnetic field, and a very good reproduction of the D st index. It is shown that the ring current is most important during intense storms, whereas the near-Earth tail currents contribute more to the D st index than the ring current during moderate storms.

Storm-time ring current: model-dependent results

“…When setting the model boundary at 10 R E and using Tsyganenko magnetic fields, we include the effect from the nearEarth tail current from the stretched magnetic field lines. The results are in agreement with the modeling efforts by Ganushkina et al (2004). They have shown that during moderate storms the tail current intensifies first and tracks the drop in the Dst index.…”

“…Several studies, however, have suggested that the Dst index contains contributions from many other sources than the azimuthally symmetric ring current (Campbell, 1973;Arykov and Maltsev, 1993;Dremukhina et al, 1999;Alexeev et al, 2001;Ohtani et al, 2001;Liemohn, 2003;Ganushkina et al, 2004;Kalegaev et al, 2005). Based on GOES 8 measurements and their correlation with Dst, Ohtani et al (2001) determined the contribution from the tail current at Dst minimum to be, at least, 25 %.…”

“…Liemohn (2003) showed that this is roughly accounted for by a systematic overestimation inherent in the DPS relation (because of the assumption that all of the plasma is within the integration volume, the DPS relation implicitly includes a truncation current). By modeling several storm events, Ganushkina et al (2004) have shown that the tail current intensifies first and tracks the drop in the Dst index. The ring current develops more slowly, and then stays at an increased level longer than the tail current.…”

Phase Space Density matching of relativistic electrons using the Van Allen Probes: REPT results

“…The utility of being able to compute the actual (rather than model) value of lies in estimating the adequacy of the model being used and the uncertainties. Multipoint magnetic field data would also allow the use of event-fitted models [e.g., Ganushkina et al, 2004] that could allow the use of the actual first invariant with consistent estimates of the second and third invariants. In the absence of operational event-fitted models, it is necessary to estimate the uncertainty in the estimated arising from the use of the empirical models.…”

Space Weather Effects Produced by the Ring Current Particles

“…However, the Dst-index contains contributions from many other sources than the azimuthally symmetric ring current and their contributions can be significant or even largest during disturbed conditions. The suggested magnetospheric sources include (1) magnetopause currents, (2) cross-tail current, (3) partial ring current, and even (4) substorm current wedge (see the review by Maltsev 2004 and references therein) and also studies by Friedrich et al (1999), Munsami (2000), Ohtani et al (2001), Liemohn et al (2001a,b), Liemohn (2003), Ganushkina et al (2004Ganushkina et al ( , 2010, Kalegaev et al (2005). The magnetic field measured on the ground contains contributions from all current systems and there is no other way to separate them but to use magnetospheric models.…”