Organization of the magnetosphere during substorms

The change in degree of organization of the magnetosphere during substorms is investigated by analyzing various geomagnetic indices, as well as interplanetary magnetic field z-component and solar wind flow speed. We conclude that the magnetosphere self-organizes globally during substorms, but neither the magnetosphere nor the solar wind become more predictable in the course of a substorm. This conclusion is based on analysis of five hundred substorms in the period from 2000 to 2002. A minimal dynamic-stochastic model of the driven magnetosphere that reproduces many statistical features of substorm indices is discussed

On the magnetospheric ULF wave counterpart of substorm onset

One near‐ubiquitous signature of substorms observed on the ground is the azimuthal structuring of the onset auroral arc in the minutes prior to onset. Termed auroral beads, these optical signatures correspond to concurrent exponential increases in ground ultralow frequency (ULF) wave power and are likely the result of a plasma instability in the magnetosphere. Here, we present a case study showing the development of auroral beads from a Time History of Events and Macroscale Interactions during Substorms (THEMIS) all‐sky camera with near simultaneous exponential increases in auroral brightness, ionospheric and conjugate magnetotail ULF wave power, evidencing their intrinsic link. We further present a survey of magnetic field fluctuations in the magnetotail around substorm onset. We find remarkably similar superposed epoch analyses of ULF power around substorm onset from space and conjugate ionospheric observations. Examining periods of exponential wave growth, we find the ground‐ and space‐based observations to be consistent, with average growth rates of ∼0.01 s−1, lasting for ∼4 min. Cross‐correlation suggests that the space‐based observations lead those on the ground by approximately 1–1.5 min. Meanwhile, spacecraft located premidnight and ∼10 RE downtail are more likely to observe enhanced wave power. These combined observations lead us to conclude that there is a magnetospheric counterpart of auroral beads and exponentially increasing ground ULF wave power. This is likely the result of the linear phase of a magnetospheric instability, active in the magnetotail for several minutes prior to auroral breakup

Generic model for magnetic explosions applied to solar flares

An accepted model for magnetospheric substorms is proposed as the basis for a generic model for magnetic explosions, and is applied to solar flares. The model involves widely separated energy-release and particle-acceleration regions, with energy transported Alfv\'enically between them. On a global scale, these regions are coupled by a large-scale current that is set up during the explosion by redirection of pre-existing current associated with the stored magnetic energy. The explosion-related current is driven by an electromotive force (EMF) due to the changing magnetic flux enclosed by this current. The current path and the EMF are identified for an idealized quadrupolar model for a flare

Transpolar voltage and polar cap flux during the substorm cycle and steady convection events

Transpolar voltages observed during traversals of the polar cap by the Defense Meteorological Satellite Program (DMSP) F-13 spacecraft during 2001 are analyzed using the expanding-contracting polar cap model of ionospheric convection. Each of the 10,216 passes is classified by its substorm phase or as a steady convection event (SCE) by inspection of the AE indices. For all phases, we detect a contribution to the transpolar voltage by reconnection in both the dayside magnetopause and in the crosstail current sheet. Detection of the IMF influence is 97% certain during quiet intervals and >99% certain during substorm/SCE growth phases but falls to 75% in substorm expansion phases: It is only 27% during SCEs. Detection of the influence of the nightside voltage is only 19% certain during growth phases, rising during expansion phases to a peak of 96% in recovery phases: During SCEs, it is >99%. The voltage during SCEs is dominated by the nightside, not the dayside, reconnection. On average, substorm expansion phases halt the growth phase rise in polar cap flux rather than reversing it. The main destruction of the excess open flux takes place during the 6- to 10-hour interval after the recovery phase (as seen in AE) and at a rate which is relatively independent of polar cap flux because the NENL has by then retreated to the far tail. The best estimate of the voltage associated with viscous-like transfer of closed field lines into the tail is around 10 kV

VLF, magnetic bay and Pi2 substorm signatures at auroral and midlatitude ground stations

A superposed epoch analysis of 100–300 substorms is performed to determine the median size and shape of the substorm-associated VLF chorus, magnetic bay, and Pi2 pulsation burst observed at the near-auroral Halley research station, Antarctica, and at the midlatitude Faraday station at three different local times (2230, 2330, 0130 MLT). The spatial and temporal properties of the magnetic bay signatures are compared with the University of York implementation of the Kisabeth–Rostoker substorm current wedge (SCW) model and the Weimer pulse model, respectively. These constitute the best analytical models of the substorm to date. It is shown that the polarities and relative amplitudes of the observed magnetic bays in the H, D, and Z components at Halley at midnight MLT and at Faraday in the premidnight sector are consistent with the York model for a SCW 3 hours wide in MLT with its westward electrojet at 67°S magnetic latitude. In particular the little-discussed Z component of the bay agrees with the model and is shown to be the clearest substorm signature of the three components, especially at midlatitude. The midnight and postmidnight bays are similar to the premidnight case but progressively smaller and cannot be fully reconciled with the model. The shape of the H and Z bays at Halley and the D bays at Faraday fit a normalized Weimer pulse well, with Weimer's 2 h−1 recovery rate, but the other components do not. The D component at Halley and H at Faraday do fit the Weimer pulse shape but with a faster recovery rate of 4 h−1. It is proposed that this is due to the effect of a decaying current in the SCW combining with the geometrical effect of changing SCW configuration and position relative to the observing station. The Z component at Faraday recovers more slowly than the 2 h−1 Weimer prediction; we cannot explain this. Secondary bays at Halley and Faraday show a clear tendency to recur after 2 hours. Inflection points just prior to onset at Halley and Faraday are argued to be related to reduced convection associated with northward turning of the IMF. The median substorm signature at Halley in the Pi2 frequency band (7–25 mHz) is well correlated with the bay structure, showing that it is part of a broader band, possibly turbulent, spectrum in the substorm-dependent DP2 current. There is evidence of a minor additional narrow band component occurring at substorm onset. This is the dominant signal at Faraday which shows the classic midlatitude substorm signature, a short Pi2 pulsation burst at onset, that decreases progressively in intensity with increasing local time, implying a source region biased to the evening side or else preferred propagation to the east from a near-midnight source

Nuclear Magnetohydrodynamic EMP, Solar Storms, and Substorms

In addition to a fast electromagnetic pulse (EMP), a high altitude nuclear burst produces a relatively slow magnetohydrodynarnic EMP (MHD EMP), whose effects are like those from solar storm geomagnetically induced currents (SS GIC). The MHD EMP electric field E < 10^-1 V/m and lasts < 10^2 sec, whereas for solar storms E > 10^-2 V/m and lasts >10^3 sec. Although the solar storm electric field is lower than MHD EMP, the solar storm effects are generally greater due to their much longer duration. Substorms produce much smaller effects than SS GIC, but occur much more frequently. This paper describes the physics of such geomagnetic disturbances and analyzes their effects.Comment: 29 pages, 14 figures, 5 table