Eftyhia Zesta

The auroral signature of earthward flow bursts observed in the magnetotail

We present direct evidence that transient Earthward flow bursts in the magnetotail can produce an observable signature in the optical aurora. This signature is north-south aligned auroral structures that are extensions of transient intensifications near the poleward boundary of the auroral oval. Our study focuses on the period from 0500 to 0700 UT on January 7, 1997, during which five distinct flow bursts are observed in the Geotail data. At that time, the spacecraft was located approximately 30 RE downtail on field lines that project down to the CANOPUS array of ground based instruments. We find that each of the flow bursts seen in the Geotail data is associated with an auroral poleward boundary intensification (PBI) observed in the CANOPUS meridian scanning photometer (MSP) data, which appears as a north-south aligned auroral structure in the CANOPUS all-sky imager (ASI) data. Based on these observations we estimate that the fast flows originated between 50 and 100 RE downtail.

Magnetometer array for cusp and cleft studies observations of the spatial extent of broadband ULF magnetic pulsations at cusp/cleft latitudes

We have used magnetometer data from 10 locations in Arctic Canada and Greenland, covering over 5 hours in magnetic local time at magnetic latitudes from 75° to 79°, to characterize the dayside patterns of enhanced long-period ULF (10- to 600-s period) wave power at cusp/cleft latitudes. We conclude the following: (1) In agreement with earlier single-station studies, we find that the most common wave type is broadband noise (Pi 1-2). Distinct Pc 3-4 activity and more sustained monochromatic Pc 5 activity are most apparent when this broadband noise is weak. (2) Multistation observations also make clear that strong, broadband Pi 1-2 signals are both temporally and spatially structured: Although their amplitude is somewhat larger near local noon and near nominal cusp latitudes, they often occur simultaneously (to within a few minutes) at all stations. They are thus not local signals, and cannot be interpreted as evidence of passage of an auroral region or boundary over an individual magnetic observatory. In particular, we have found no evidence for a distinctive “cusp” signature in broadband ULF waves in this frequency range. (3) The occurrence of strong broadband Pi 1-2 signals at these latitudes appears to be controlled largely by solar wind velocity. We found good correlations between the occurrence of strong Pi 1-2 signals and high solar wind velocity, and we also noted some dependence on the cone angle of the interplanetary magnetic field for moderate to low solar wind velocities. We speculate that there may be an additional dependence on enhanced levels of trapped plasma in regions topologically connected to the very high latitude dayside ionosphere, such as the entry layer, high-latitude dayside field minimum regions, or plasma mantle. Available satellite data on the level of trapped energetic electron fluxes at geosynchronous orbit showed that broadband power levels appeared to correlate with enhanced flux levels on the time scale of days, but not on shorter time scales, suggesting that any such dependence is not directly related to substorm injections.

Magnetospheric reconnection driven by solar wind pressure fronts

Abstract. Recent work has shown that solar wind dynamic pressure changes can have a dramatic effect on the particle precipitation in the high-latitude ionosphere. It has also been noted that the preexisting interplanetary magnetic field (IMF) orientation can significantly affect the resulting changes in the size, location, and intensity of the auroral oval. Here we focus on the effect of pressure pulses on the size of the auroral oval. We use particle precipitation data from up to four Defense Meteorological Satellite Program (DMSP) spacecraft and simultaneous POLAR Ultra-Violet Imager (UVI) images to examine three events of solar wind pressure fronts impacting the magnetosphere under two IMF orientations, IMF strongly southward and IMF Bz nearly zero before the pressure jump. We show that the amount of change in the oval and polar cap sizes and the local time extent of the change depends strongly on IMF conditions prior to the pressure enhancement. Under steady southward IMF, a remarkable poleward widening of the oval at all magnetic local times and shrinking of the polar cap are observed after the increase in solar wind pressure. When the IMF Bz is nearly zero before the pressure pulse, a poleward widening of the oval is observed mostly on the nightside while the dayside remains unchanged. We interpret these differences in terms of enhanced magnetospheric reconnection and convection induced by the pressure change. When the IMF is southward for a long time before the pressure jump, open magnetic flux is accumulated in the tail and strong convection exists in the magnetosphere. The compression results in a great enhancement of reconnection across the tail which, coupled with an increase of magnetospheric convection, leads to a dramatic poleward expansion of the oval at all MLTs (dayside and nightside). For near-zero IMF Bz before the pulse the open flux in the tail, available for closing through reconnection, is smaller. This, in combination with the weaker magnetospheric convection, leads to a more limited poleward expansion of the oval, mostly on the nightside. Key words. Magnetospheric physics (solar windmagnetosphere interactions; magnetospheric configuration and dynamics; auroral phenomena)

Auroral disturbances during the January 10, 1997 magnetic storm

It is well known that intense and frequent auroral-zone disturbances, often attributed to substorms, occur during magnetic storms. We examine observations during the January 10, 1997 main phase and find that observed auroral-zone activity was dominated by a combination of global auroral and current enhancements, which are a direct response to solar wind dynamic pressure enhancements, and poleward boundary intensifications, which are localized in longitude and have an auroral signature that moves equatorward from the magnetic separatrix. Poleward and azimuthally expanding regions of auroral activity which accompany substorms are found to contribute significantly less to the observed activity. This suggests that poleward boundary intensifications and dynamic pressure responses may be an important cause of disturbances during periods of enhanced convection such as magnetic storms and convection bays.

Propagation of the preliminary reverse impulse of sudden commencements to low latitudes

It has been thought that the preliminary reverse impulse (PRI) of the sudden commencements (SC) phenomena occurs simultaneously on the ground at different locations. A popular explanation is that the PRI propagates through the Earth-ionosphere waveguide at the 35 ground magnetometer stations during the SC event on September 24, 1998, and found clear differences in the arrival time of PRI. We calculated the MHD wave propagation time from the location of the first compression of the magnetosphere to the low-latitude ground stations and found good agreement with the observed PRI arrival times. Our calculation also indicates that the wavefront is seriously distorted by the inhomogeneity of the magnetosphere and the small difference in PRI arrival time between high-latitude and low-latitude observations cannot be an indicator of a super-Alfvenic propagation. We also found implications that high-latitude PRIs can be induced by the vortex of ionospheric currents at nearby latitudes, and the motion of the current vortex can affect the arrival time of high-latitude PRIs.

Statistical study of effect of solar wind dynamic pressure enhancements on dawn‐to‐dusk ring current asymmetry

[1]In this paper, we statistically investigate the effect of solar wind dynamic pressure enhancements on the dawn-to-dusk ring current asymmetry by examining disturbances of the ASY-H index and low-latitude and midlatitude ground asymmetric perturbations in the north-south (H) component of the geomagnetic field during 186 events occurring from 1 June 2003 to 30 September 2004. Both storm time and nonstorm time events are included. It is found that a pressure enhancement further intensifies the ring current asymmetry provided that the ring current is already asymmetric at the time of the onset of the pressure enhancement. This effect strongly depends on the IMF B z conditions prior to the pressure enhancement. Generally, for negative IMF B z , pressure enhancements further increase the ring current asymmetry. This effect also depends on the strength of the pressure enhancement. Under the same IMF B z conditions, the stronger the pressure enhancement is, the stronger the intensification of the asymmetric ring current is. The IMF B z conditions during a pressure enhancement play a similar role to that of the IMF B z preconditioning. The results further show that midlatitude H perturbations around the local noon or midnight region as well as the ASY-H index often include significant contribution from field-aligned currents, e.g., the region 1 (R1) or region 2 (R2) currents or the substorm current wedge. Citation:Shi, Y., E. Zesta, L. R. Lyons, K. Yumoto, and K. Kitamura (2006), Statistical study of effect of solar wind dynamic pressure enhancements on dawn-to-dusk ring current asymmetry,J. Geophys. Res.,111, A10216, doi:10.1029/2005JA011532.

The November 9, 1993, traveling convection vortex event: A case study

On November 9, 1993, at around 1715 UT, a strong and well-structured traveling convection vortex (TCV) event occurred and was observed by the Magnetometer Array for Cusp and Cleft Studies (MACCS) and the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) magnetometer chains. The Greenland chain, which is located farther to the east, observed only a very weak signature for the same event. We studied the propagation characteristics of the two-dimensional vortical TCV current patterns. The TCV event is a series of four vortical currents propagating westward. We found that individual vortices propagate with different speeds. The vortices are created in the early postnoon region. They accelerate as they strengthen, and some decelerate, weaken, and disappear within the 6 hours of magnetic local time of the field of view of the ground magnetometers. The strongest, main vortex of the event accelerates until it moves out of the field of view and, more than likely, reaches well into the nightside. We studied the correlated solar wind and IMF signatures as observed by the IMP 8 spacecraft, sitting in the far dawnside and outside the bow shock. We found that the transient currents in the ionosphere are the result of sharp, short-lived pressure pulses hitting the magnetopause during times of quiet and northward IMF that is primarily radial. We find that the pressure pulses are more than likely created just upstream of the bow shock by the interaction of the quasi-parallel shock with IMF orientation changes and are not intrinsic features of the upstream solar wind. We also analyze the transient signatures in the inner magnetosphere by studying the magnetic field data in the GOES 6 and 7 satellites. We suggest that a series of five successive compression and depression peaks in the GOES magnetic field data are well correlated with the set of solar wind pressure pulses. We observe a propagation velocity of the transient event from the GOES 6 spacecraft to the GOES 7 spacecraft that agrees well with the propagation velocities that we calculate from the ground magnetometer stations.

Timing of substorm signatures during the November 24, 1996, Geospace Environment Modeling event

An excellent data set has been gathered for the November 24, 1996, Geospace Environment Modeling substorm interval that included an ∼95 min interval of strongly southward interplanetary magnetic field. There were two expansion phase onsets and a pseudobreakup during this period. For both onsets the classical signatures of onset in the auroral ionosphere (ground magnetometer signatures of electrojet formation, ground observations of Pi2 pulsations, and auroral brightening) all occurred within less than a minute of each other, indicating consistency between these onset indicators and giving reliable identification of onset times. On the other hand, low-latitude positive bay observations that were most likely made near or within the longitude range of substorm onset did not provide accurate or consistent onset timing. Low-latitude Pi2s identified both onsets, though one was delayed by l min from the auroral zone onset. Our most important and unexpected result is that particle injection at synchronous orbit was observed to initiate ∼2.5 min prior to both expansion onsets in the auroral ionosphere. We suggest that the early detection of substorm onset at synchronous orbit was at least in part due to the unusually low latitude (63° magnetic) of the auroral zone onsets, which maps closer to synchronous orbit than usual. Onsets more often occur at somewhat higher latitudes, which map to a few RE beyond synchronous orbit. As a result, particle injections at synchronous orbit are generally delayed with respect to the time of current wedge initiation within the plasma sheet. We do not currently know whether or not onset within the near-Earth plasma sheet generally occurs ∼2.5 min prior to expansion onset in the auroral ionosphere. While we have no reason to believe that the timing we have observed is unique, other studies of low-latitude substorm onsets will be needed to ascertain the generality of this result.

Temporal evolution of the transpolar potential after a sharp enhancement in solar wind dynamic pressure

[1] Recent studies of ionospheric convection have shown that sudden enhancements in solar wind dynamic pressure have significant effect on the transpolar potential and the coupling efficiency between the solar wind and the terrestrial magnetosphere. Super Dual Auroral Radar Network observations of the dayside convection have demonstrated that the strength of convection correlates well with solar wind dynamic pressure variations, implying an enhancement of dayside reconnection induced by changes in solar wind pressure. At the same time, dynamic pressure increases have been shown to lead to closing of the polar cap, particularly on the nightside, and thus directly drive enhanced tail reconnection. The enhanced dayside and nightside reconnection potentials can both lead to changes in the transpolar potential, but their individual contributions and the balance between the two is not known. We present a case study of the transpolar potential evolution after a long-lasting solar wind pressure step increase. We show that the potential first rises in response to the increase in pressure, then gradually subsides a few hours later despite the solar wind pressure remaining high. We interpret this behavior in terms of pressure-driven changes in dayside and nightside reconnection.

Observations of ULF wave related equatorial electrojet and density fluctuations

Global magnetospheric Ultra Low Frequency (ULF) pulsations with frequencies in the Pc 4–5 range (f = 1.0–8 mHz) have been observed for decades in space and on Earth. ULF pulsations contribute to magnetospheric particle transport and diffusion and play an important role in magnetospheric dynamics. However, only a few studies have been performed on ionospheric observations of ULF wave-related perturbations in the vicinity of the equatorial region. In this paper we report on Pc5 wave related electric field and thus vertical drift velocity oscillations at the equator as observed by ground magnetometers and radar. We show that the magnetometer estimated equatorial ExB drift oscillate with the same frequency as ULF Pc5 waves, creating significant ionospheric density fluctuations. For independent confirmation of the vertical drift velocity fluctuation, we used JULIA 150 km radar drift velocities and found similar fluctuation with the period of 8–10 minutes. We also show ionospheric density fluctuations during the period when we observed ULF wave activities. All these demonstrate that the Pc5 wave can penetrate to the equatorial ionosphere and modulate the equatorial electrodynamics. Finally, in order to detect the ULF activities both on the ground and in space, we use ground-based magnetometer data from African Meridian B-field Education and Research (AMBER) and the South American Meridional B-field Array (SAMBA). From space, we use magnetic field observations from the GOES 12 and the Communication/Navigation Outage and Forecast System (C/NOFS) satellites. Using the WIND spacecraft as the upstream solar wind monitor, we present direct evidence that solar wind number density and ram pressure fluctuations observed far upstream from the terrestrial magnetosphere are the main drivers of ULF wave activity inside the magnetosphere. Finally, we show that the ULF waves in the same frequency range are observed in the magnetosphere by the geosynchronous GOES spacecraft, in the ionosphere by the equatorial C/NOFS satellite, and on the ground by ground-based magnetometers, indicating that the magnetospheric origin ULF wave can penetrate to the ground equatorial region and modulate the equatorial electrodynamics.