Electrodynamics of an omega-band as deduced from optical and magnetometer data

Abstract

Abstract.We investigate an omega-band event that took place above northern Scandinavia around 02:00-02:30 UT on 9 March 1999. In our analysis we use ground based magnetometer, optical and riometer measurements together with satellite based optical images. The optical and riometer data are used to estimate the ionospheric Hall and Pedersen conductances, while ionospheric equivalent currents are obtained from the magnetometer measurements. These data sets are used as input in a local KRM calculation, which gives the ionospheric potential electric field as output, thus giving us a complete picture of the ionospheric electrodynamic state during the omega-band event.The overall structure of the electric field and field-aligned current (FAC) provided by the local KRM method are in good agreement with previous studies. Also the E×B drift velocity calculated from the local KRM solution is in good qualitative agreement with the plasma velocity measured by the Finnish CUTLASS radar, giving further support for the new local KRM method. The high-resolution conductance estimates allow us to discern the detailed structure of the omega-band current system. The highest Hall and Pedersen conductances, ∼50 and ∼25 S, respectively, are found at the edges of the bright auroral tongue. Inside the tongue, conductances are somewhat smaller, but still significantly higher than typical background values. The electric field shows a converging pattern around the tongues, and the field strength drops from ∼40 mV/m found at optically dark regions to ∼10 mV/m inside the areas of enhanced conductivity. Downward FAC flow in the dark regions, while upward currents flow inside the auroral tongue. Additionally, sharp conductance gradients at the edge of an auroral tongue are associated with narrow strips of intense FACs, so that a strip of downCorrespondence to: H. Vanhamäki (heikki.vanhamaki@fmi.fi) ward current flows at the eastern (leading) edge and a similar strip of upward current is present at the western (trailing) edge. The Joule heating follows the electric field pattern, so that it is diminished inside the bright auroral tongue.

Omega band pulsating auroras observed onboard THEMIS spacecraft and on the ground

“…Originally, the name referred to the distinct, undulating shape of the auroral arc, which resembles an inverted Greek letter Ω. Omega bands, which are typically 400–1000 km in size, are usually observed propagating eastward at speeds of 0.4–2 km/s in the morning sector auroral zone and are generally associated with the recovery phase of magnetospheric substorms. The ionospheric electrodynamics of omega bands has been reported in several papers [see e.g., Wild et al ., , ; Syrjäsuo and Donovan , ; Amm et al ., ; Vanhamäki et al ., , and references therein], all of which support the overall picture of a sequence of upward and downward field‐aligned currents located in the bright and dark regions of the structure, respectively. As the band structure with its temporally stationary current system moves above ground‐based magnetometers, Ps6‐type magnetic pulsations (period 4–40 min) are observed.…”

Midnight sector observations of auroral omega bands

“… Lühr and Schlegel [1994] described omega bands as “a luminous band from which tongue‐like protrusions extend toward the north” with the bright tongues shaped like a Greek Ω and the dark area separating adjacent tongues shaped like an inverted Ω. In recent research, the term omega band has been used to described all of the above variants on what is assumed to be the same basic auroral structure [ Syrjäsuo and Donovan , 2004; Safargaleev et al , 2005; Vanhamäki et al , 2009].…”

“…Omega bands and magnetic pulsations in the Ps6 wave band (4–40 min periodicity) are usually observed simultaneously [ Kawasaki and Rostoker , 1979; André and Baumjohann , 1982], with magnetic disturbances interpreted as evidence of the passage of field‐aligned currents within the auroral structures [ Lühr and Schlegel , 1994; Wild et al , 2000]. Omega bands, typically 400–1000 km in size, are usually observed propagating eastward (i.e., dawnward) at speeds of 0.4–2 km s −1 in the morning sector auroral zone and are generally associated with the recovery phase of magnetospheric substorms [e.g., Vanhamäki et al , 2009, and references therein].…”

“…For example, whereas Akasofu and Kimball's omega bands were distorted arcs, Lyons and Walterscheid [1985] presented observations of omega bands with a dark, inverted W shape formed by bright torches extending poleward from the auroral oval, and Opgenoorth et al [1994] reported "streets" of multiple omega band structures in which undulations on the poleward boundary gave rise to alternating bright humps and dark bays. Lühr and Schlegel [1994] described omega bands as "a luminous band from which tongue-like protrusions extend toward the north" with the bright tongues shaped like a Greek W and the dark area separating adjacent tongues shaped like an inverted W. In recent research, the term variants on what is assumed to be the same basic auroral structure [Syrjäsuo and Donovan, 2004;Safargaleev et al, 2005;Vanhamäki et al, 2009].…”

Electrodynamic structure of the morning high‐latitude trough region

“…Magnetic data from the IMAGE magnetometer network are used in calculating the equivalent current, similar to many previous studies [e.g., Vanhamäki et al , ]. The ground magnetic measurements are converted into two‐dimensional (latitude‐longitude) maps of ionospheric equivalent currents with the SECS method, introduced by Amm and Viljanen [] and carefully tested by Pulkkinen et al [].…”