Dynamics of the inner magnetosphere near times of substorm onsets

Abstract

The electrodynamics of the inner magnetosphere near times of substorm onsets have been investigated using CRRES measurements of magnetic and electric fields, energetic electron fluxes, in conjunction with ground‐based observations. Six events were studied in detail, spanning the 2100 to 0000 MLT sector and L values from 5 to 7. In each case the dawn‐dusk electric field was enhanced over typical background electric fields, and significant, low‐frequency pulsation activity was observed. The amplitudes of the pulsations were larger than the background electric fields. Dusk‐dawn excursions of the cross‐tail electric field often correlated with changes in currents and particle energies at CRRES and with ULF wave activity observed on the ground. Variations of the electric field and Poynting vectors with periods in the Pi 2 range are consistent with bouncing AlfVén waves that provide electromagnetic communication between the ionosphere and plasma sheet. Magnetic signatures of field‐aligned current filaments directed away from the ionosphere, presumably associated with the substorm current wedge, were observed during three orbits. In all cases, ground signatures of substorm expansion were observed at least 5 min before the injection of electrons at CRRES. Field‐aligned fluxes of counter‐streaming, low‐energy electrons were detected after three of the injections. We develop an empirical scenario for substorm onset. The process grows from ripples at the inner edge of the plasma sheet associated with dusk‐dawn excursions of the electric field, prior to the beginning of dipolarization. Energy derived from the braking of the inward plasma convection flows into the ionosphere in the form of Poynting flux. Subsequently reflected Poynting flux plays a crucial role in the magnetosphere‐ionosphere coupling. Substorms develop when significant energy (positive feedback?) flows in both directions, with the second cycle stronger than the initial. Pseudobreakups occur when energy flow in both directions is weak (negative feedback?). “Explosive‐growth‐phase” signatures occur after onset, early in the substorm expansion phase. Heated electrons arrive at the spacecraft while convection is earthward, during or at the end of electromagnetic energy flow away from the ionosphere.

Polar spacecraft based comparisons of intense electric fields and Poynting flux near and within the plasma sheet‐tail lobe boundary to UVI images: An energy source for the aurora

“…Over (Maynard et al, 1996). During the onset of one intense substorm in that study, the net Poynting flux due to Alfven waves was directed toward the Earth along the ambient magnetic field and ranged up to 0.8 ergs cm-2s -• for several minutes.…”

Role of substorm-associated impulsive electric fields in the ring current development during storms

“…Observations show that substorm-associated electric fields usually display a very complicated behavior (Maynard et al, 1996). Large, transient electric fields appear in the plasma sheet during the substorm expansion phase (Aggson et al, 1983;Cattell and Mozer, 1984;Rowland and Wygant, 1998; 580 N. Yu.…”

Period and damping factor of Pi2 pulsations during oscillatory flow braking in the magnetotail

“…Hence, midlatitude Pi2s could also be driven by field line resonances [e.g., Takahashi et al, 1996;Glassmeier et al, 1999] from compressional waves that move radially inward Chang and Lanzerotti, 1975], with following injection in the inner magnetosphere. The onset of the compressional waves could be quasi-periodic plasma variations near the leading edge of the plasma sheet or in the injected plasma [Smits et al, 1986;Saka et al, 1996;Maynard et al, 1996]. This may be the reason that midlatitude Pi2s are observed more globally (though with smaller amplitudes) than the plasma sheet oscillating flows (which are localized within BBF channels 2-3 R E wide).…”