Interplanetary magnetic field control of dayside auroral activity and the transfer of momentum across the dayside magnetopause

Physical processes producing magnetospheric substorms and magnetic storms.

[1] Large-scale traveling ionospheric disturbances (LSTIDs) were statistically studied using total electron content (TEC) data from the GPS Earth observation network (GEONET) in Japan during 45 months from April 1999 to December 2002. One hundred fifty-four LSTIDs propagating southward were identified with time sequences of two-dimensional TEC maps. Fifty LSTIDs were observed in 2000, 45 in 2001, and 38 in 2002. Their occurrence rate (occurrence probability of one LSTID per 3 hours) increased as Kp value increased, that is, 1% at Kp = 4 and 75% at Kp = 9. The disturbed-time LSTIDs were frequently observed over Japan in spring and autumn, which is consistent with the seasonal dependence of the geomagnetic disturbances. On the other hand, the number of LSTIDs under quiet conditions, Kp ≤ 3, reached 43, that is, 28% of all the LSTIDs. The wave parameters, such as the damping rate, horizontal velocity and direction, and period of the clear 58 LSTIDs were precisely determined. The amplitude of the quiet-time LSTIDs decreased during their passage over Japan for every event, while that of the disturbed-time LSTIDs not only decreased but also increased. The LSTIDs can be classified from these observational results into the following three types: the disturbed-time damping LSTIDs (DD-LSTIDs), the disturbed-time growing LSTIDs (DG-LSTIDs), and the quiet-time damping LSTIDs (QD-LSTIDs). The occurrence number of DD-LSTIDs, DG-LSTIDs, and QD-LSTIDs was 35 (60%), 11 (19%), and 12 (21%), respectively. The mean horizontal velocity, period, wavelength, and propagation direction of all LSTIDs were 475 ± 171 m/s, 80 ± 29 min, 2131 ± 863 km, and 3 ± 19° east from south, respectively. Both the growth and decrease rates of the LSTIDs were correlated with their propagation velocities. An examination of the relation between the damping rates and the vertical propagation direction, θ, of atmospheric gravity waves (AGWs) which was derived from the horizontal velocity and the period of LSTIDs using the AGWs' dispersion relation, revealed that their damping and growth rates showed a clear correlation with θ. This correlation is due to the ion-drag effect, which is directly dependent on the angle between AGWs' propagation directions and the geomagnetic field. Considering the inclination of the geomagnetic field over Japan, both the damping and growing LSTIDs could be explained by the upward and downward propagating AGWs, respectively. The QD-LSTIDs had smaller θ which resulted from slower velocities and longer periods than those for the disturbed-time LSTIDs. These different characteristics would reflect those of the source mechanisms of the three types of LSTIDs.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2003JA010302

A statistical study of large-scale traveling ionospheric disturbances using the GPS network in Japan

7 – Physical Processes Producing Magnetospheric Substorms and Magnetic Storms

The generation of flow and current vortices in the dayside auroral ionosphere has been predicted for two processes ocurring at the dayside magnetopause. The first of these mechanisms is time-dependent magnetic reconnection, in “flux transfer events” (FTEs); the second is the action of solar wind dynamic pressure changes. The ionospheric flow signature of an FTE should be a twin vortex, with the mean flow velocity in the central region of the pattern equal to the velocity of the pattern as a whole. On the other hand, a pulse of enhanced or reduced dynamic pressure is also expected to produce a twin vortex, but with the central plasma flow being generally different in speed from, and almost orthogonal to, the motion of the whole pattern. In this paper, we make use of this distinction to discuss recent observations of vortical flow patterns in the dayside auroral ionosphere in terms of one or other of the proposed mechanisms. We conclude that some of the observations reported are consistent only with the predicted signature of FTEs. We then evaluate the dimensions of the open flux tubes required to explain some recent simultaneous radar and auroral observations and infer that they are typically 300 km in north–south extent but up to 2000 km in longitudinal extent (i.e., roughly 5 hours of MLT). Hence these observations suggest that recent theories of FTEs which invoke time-varying reconnection at an elongated neutral line may be correct. We also present some simultaneous observations of the interplanetary magnetic field (IMF) and solar wind dynamic pressure (observed using the IMP8 satellite) and the ionospheric flow (observed using the EISCAT radar) which are also only consistent with the FTE model. We estimate that for continuously southward IMF ( ≈ 5 nT) these FTEs contribute about 30 kV to the mean total transpolar voltage (∼30%).

/pdf/the-ionospheric-signatures-of-flux-transfer-events-and-solar-1ornfe571t.pdf

The ionospheric signatures of flux transfer events and solar wind dynamic pressure changes

A special campaign was conducted at the Sondre Stromfjord incoherent scatter radar facility in October 1985 to study hydromagnetic and atmospheric gravity were phenomena near the cusp regions of the magnetosphere. During a day when the convection reversal boundary was measured just to the north of the radar, a field-aligned current filament of inferred amplitude --2 x 10V A was measured at the boundary. Both ion and electron heating appeared to accompany the current filament. Magnetic field data acquired in the southern conjugate hemisphere are used to conclude that the current filament was probably on closed field lines. The current filament may be evidence of the magnetosphere boundary layer on closed field lines, perhaps produced by the injection of magnetosheath plasmoids onto the field lines by a flux transfer-type reconnection process. copyright American Geophysical Union

Ionosphere and ground‐based response to field‐aligned currents near the magnetospheric cusp regions

Moderate geomagnetic activity followed by a large scale TID was observed in Europe and eastern North America near 1200 UT on October 18, 1985 during the Worldwide Atmospheric Gravity Wave Study. The speed and direction of the TID are estimated, suggesting that it was caused by an atmospheric gravity wave expanding from a localized source over the Arctic Ocean. Auroral imaging shows that the source region was located near the westward edge of an expanding auroral bulge and may have been associated with a westward traveling surge. The TID periods increased with distance from the source region and the largest TID amplitudes were seen along a line perpendicular to the orientation of the auroral oval.

An observation of atmospheric gravity wave cause and effect during the October 1985 WAGS campaign

Studies of a cusp-region hydromagnetic wave event with magnetometers and incoherent scatter radar

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Ionosphere and ground‐based response to field‐aligned currents near the magnetospheric cusp regions

An observation of atmospheric gravity wave cause and effect during the October 1985 WAGS campaign

Studies of a cusp-region hydromagnetic wave event with magnetometers and incoherent scatter radar