Boundary Of Aurora Research Articles

Strong turbulent flow in the subauroral region in the Antarctic can deteriorate satellite-based navigation signals

In the subauroral zone at the boundary of the auroral oval in the evening and night hours during geomagnetic disturbances, a narrow (about 1°–2°) and extended structure (several hours in longitude) is formed. It is known as a polarization jet (PJ) or the subauroral ion drift (SAID). The PJ/SAID is a fast westward ion drift and is one of the main signatures of a geomagnetic disturbance in the subauroral ionosphere at the altitudes of the F-layer, when the geomagnetic AE index reaches more than 500 nT. Plasma speed in the PJ/SAID can reach several kilometres per second, and the size of plasma irregularities inside it can reach scales from tens of meters to several hundred meters. Such high velocities and structured plasma can affect trans-ionospheric radio waves and lead to scintillations in the received signal. We show that at the moment of auroral activity intensification, an increase in the magnitude of phase scintillation index (σϕ) as well as loss of satellite signals lock were observed in the region of the PJ/SAID equatorward of the auroral oval over Dronning Maud Land (Queen Maud Land) in Antarctica. We find that fluctuations inside the PJ/SAID can lead to serious deterioration of radio communication or navigational services. We emphasize the importance of considering the geometry of the beam passing from the GNSS satellite to the receiver on the ground. We highlight the mutual contribution of the PJ/SAID and the diffuse aurora boundary, which are almost impossible to separate in practice. Our results demonstrate the importance of considering the subauroral zone, where very dynamic plasma formations can occur with a strong flow and various-scale irregularities inside that lead to serious interference in satellite communications.

Transient Oscillations Near the Dayside Open‐Closed Boundary: Evidence of Magnetopause Surface Mode?

AbstractGeomagnetic pulsations in Pc5‐6 band (~3–20 min) are persistent feature of ULF activity at dayside high latitudes. Magnetopause surface eigenmodes may be suggested as potential mechanism of these pulsations. One might expect the ground response of these modes to be near ionospheric projection of the open‐closed field line boundary (OCB). Using data from instruments located at Svalbard we study transient geomagnetic response to impulsive “intrusion” of magnetosheath plasma into the dayside magnetosphere. These intrusions are triggered by modest changes of interplanetary magnetic field to southward, and observed as sudden shifts of equatorward red aurora boundary to lower latitudes and green line emission intensification. Each auroral disturbance is accompanied by burst of ~1.7–2.0‐mHz geomagnetic pulsations. Near‐cusp latitudinal structure of ULF pulsations is compared with instant location of equatorward boundary of the red aurora, assumed to be a proxy of the OCB. Optical OCB latitude has been identified using data from the meridian scanning photometer. The latitudinal maximum of the transient geomagnetic response tends to be located near disturbed OCB proxy, within the error ~1°–2° of the photometer and magnetometer methods. Recorded transient pulsations may be associated with the ground image of the magnetopause surface mode harmonic. Theoretical consideration indicates that after an initial excitation, surface large‐scale mode converts into localized Alfvén oscillations and thus can exist for limited time only. Therefore, MHD surface modes in realistic inhomogeneous plasma cannot be considered in isolation, but as a combined system of modes with discrete and continuous spectra with irreversible transformation between them.

Empirical Modeling of the Equatorward Boundary of Auroral Precipitation Using DMSP and DE 2

AbstractA new model of the equatorward boundary of the diffuse aurora has been developed using observations of precipitating particles made by the Defense Meteorological Satellite Program (DMSP) from 1987 to 2012 as well as Dynamics Explorer 2 (DE 2), which operated from August 1981 to February 1983. Using a local multilinear regression algorithm, we investigated the use of different combinations of magnetometer indices and solar wind coupling functions with different averaging periods and weights to find the best parameterization for a model of the equatorward boundary of the aurora. We find that weighted averages of the AE index and the solar wind coupling function dΦMP/dt both outperform the often used Kp index. Using conjunction events where two DMSP satellites cross the auroral boundary at nearly the same time, we show that these models are better at extrapolating the auroral boundary to different local times than previous models.

Statistical study of auroral omega bands

Abstract. The presence of very few statistical studies on auroral omega bands motivated us to test-use a semi-automatic method for identifying large-scale undulations of the diffuse aurora boundary and to investigate their occurrence. Five identical all-sky cameras with overlapping fields of view provided data for 438 auroral omega-like structures over Fennoscandian Lapland from 1996 to 2007. The results from this set of omega band events agree remarkably well with previous observations of omega band occurrence in magnetic local time (MLT), lifetime, location between the region 1 and 2 field-aligned currents, as well as current density estimates. The average peak emission height of omega forms corresponds to the estimated precipitation energies of a few keV, which experienced no significant change during the events. Analysis of both local and global magnetic indices demonstrates that omega bands are observed during substorm expansion and recovery phases that are more intense than average substorm expansion and recovery phases in the same region. The omega occurrence with respect to the substorm expansion and recovery phases is in a very good agreement with an earlier observed distribution of fast earthward flows in the plasma sheet during expansion and recovery phases. These findings support the theory that omegas are produced by fast earthward flows and auroral streamers, despite the rarity of good conjugate observations.

VIIRS Captures Aurora Motions

It has been found that the day/night band of the Visible Infrared Imaging Radiometer Suite is capable of observing rapid motions of the aurora. The images that led to this discovery are shown. Shifts in the apparent position of the aurora boundary between consecutive scans of the instrument, which occur ~1.79 s apart, allow the cross-track relative speed of the aurora to be calculated. The physical basis for these observations and the method for determining the speed of auroral motions are discussed. These new satellite observations compare favorably with ground-based measurements presented in previous studies.

Nightside polar rain aurora boundary gap and its applications for magnetotail reconnection

[1] Polar rain electrons with energy around 1 keV occasionally fill the polar cap ionosphere. Their energy flux is high enough to produce detectable auroral emissions, called polar rain aurora (PRA). We report a PRA event on 2 August 2002 and the estimation of the magnetotail reconnection location and size from the PRA boundary. The distinguishing feature of this event identified from the TIMED/GUVI data is an emission depletion gap between PRA and auroral oval. The size of the gap is about 2° in latitude and extended from the dusk sector (17:00 MLT) to the premidnight sector (22:00 MLT). This gap coincides with the electron flux gap in the DMSP electron energy flux data. The electron flux data show the existence of energy dispersion at the gap. These GUVI and DMSP observations support the idea that the gap is very likely due to a magnetic reconnection in the magnetotail that blocks the solar wind electron entry into the polar ionosphere. Based on the electron energy dispersion, the nightside X line is estimated to be at X = −50 to −60 RE, which is consistent with results from other studies. The gap extends from the nightside to the duskside and presumably to the dawnside. This new information suggests that the extension of the tail X line over the entire plasma sheet in the dawn-dusk direction under a strongly southward IMF (Bz ∼ −10 nT) and the curved nature of the X line.

Systematics in auroral energy spectra

Observation of a systematic increase in the mean energy of auroral electrons as the total precipitated energy increases. The change in 4278-A N2(+) intensity from about 50 R to about 10 kR correlates better with the change in mean energy of the precipitating electrons than with the change in number flux. Nightside aurora that exhibit this property superimpose on a wide region of soft (less than or about equal to 1 keV) precipitation that extends from the equatorial boundary of aurora to well poleward of normal visible aurora. On the dayside, the soft 'cusp' precipitation generates the statistical oval, although there may be larger energy inputs equatorward of the oval.