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At least 451 records · Page 25
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NASA NTRS2005

Interball-1 Observations of Mantle FTEs

Flux transfer events (FTEs) are common on the Earth's dayside low- and mid-latitude magnetopause, where they tend to occur for southward interplanetary magnetic field (IMF) orientations. "Direct" events exhibiting (+,-) bipolar signatures normal to the nominal magnetopause predominate at northern latitudes, whereas reverse events exhibiting (-,+) bipolar signatures normal to the nominal magnetopause predominate at southern latitudes. This paper reports Interball-1 magnetometer observations of reverse events on the high-latitude northern magnetopause and direct events on the high-latitude southern magnetopause. During one sequence of events, the sense of the bipolar signature normal to the nominal magnetopause reversed. In conjunction with the fact that most events occur when and where magnetosheath and magnetospheric magnetic field lie nearly antiparallel, the observations suggest that lobe events are generated locally on the high-latitude magnetopause.

Sibeck, D. G.↗
NASA NTRS2006

Solar Wind Excitation of Pc5 Fluctuations in the Magnetosphere and on the Ground

The primary purpose of this paper is to show the strong link between solar wind compressional fluctuations in the 1-8 mHz frequency range and Pc5 fluctuations in the magnetosphere near the magnetopause, at geosynchronous orbit, over the poles, and on the ground. We focus on a time interval in March and April 2002 when there was a favorable alignment of satellites combined with ten high speed solar wind streams. We used the S3C Great Observatory: specifically, ACE and Wind in the solar wind, Geotail near the magnetopause, GOES 8 and 10 at geosynchronous orbit, Cluster over the poles, and CANOPUS/CARISMA ground stations near the footpoints of magnetic field lines connected to either the magnetopause or the GOES satellites. Using four examples and a statistical survey we show that magnetospheric Pc5 fluctuations exist regardless of IMF orientation and for a wide range of speeds and dynamic pressures; the amplitude and power of magnetospheric fluctuations depends primarily on the amplitude and power of solar wind dynamic pressure fluctuations. The driving and response frequency of these geoeffective fluctuations is in the range 0.5 - 4 mHz. The most striking magnetospheric response occurs when the solar wind speed, dynamic pressure, and dynamic pressure fluctuations all increase at approximately the same time, as frequently occurs near the leading edge of high speed streams. We show evidence of oscillating Poynting Flux at the magnetopause determined using Geotail data that both excites a FLR and propagates evanescently inward. These observations suggest that, at least for this time interval at spring equinox, the entry path is from the dayside equatorial magnetopause inward; multiple field line resonances may be excited from the magnetopause to geosynchronous orbit by propagating compressional waves, with the power decreasing inward away from the magnetopause.

Kessel, Ramona L.↗
NASA NTRS2011

Concerning the Motion and Orientation of Flux Transfer Events Produced by Component and Antiparallel Reconnection

We employ the Cooling et al. (2001) model to predict the location, orientation, motion, and signatures of flux transfer events (FTEs) generated at the solstices and equinoxes along extended subsolar component and high ]latitude antiparallel reconnection curves for typical solar wind plasma conditions and various interplanetary magnetic field (IMF) strengths and directions. In general, events generated by the two mechanisms maintain the strikingly different orientations they begin with as they move toward the terminator in opposite pairs of magnetopause quadrants. The curves along which events generated by component reconnection form bow toward the winter cusp. Events generated by antiparallel reconnection form on the equatorial magnetopause during intervals of strongly southward IMF orientation during the equinoxes, form in the winter hemisphere and only reach the dayside equatorial magnetopause during the solstices when the IMF strength is very large and the IMF points strongly southward, never reach the equatorial dayside magnetopause when the IMF has a substantial dawnward or duskward component, and never reach the equatorial flank magnetopause during intervals of northward and dawnward or duskward IMF orientation. Magnetosheath magnetic fields typically have strong components transverse to events generated by component reconnection but only weak components transverse to the axes of events generated by antiparallel reconnection. As a result, much stronger bipolar magnetic field signatures normal to the nominal magnetopause should accompany events generated by component reconnection. The results presented in this paper suggest that events generated by component reconnection predominate on the dayside equatorial and flank magnetopause for most solar wind conditions.

Sibeck, D. G.↗
NASA NTRS1992

Magnetospheric plasma flows associated with boundary waves and flux transfer events

We describe plasma flow perturbations in the outer magnetosphere during the passage of magnetopause boundary waves and cylindrically shaped flux transfer events (FTEs). Spacecraft which remain within the magnetosphere observe bipolar flows normal to the nominal magnetopause accompanied by flows nearly opposite to the direction of boundary wave and/or FTE motion. Flows are generally discontinuous across FTE and magnetopause boundaries, with reversals in the component of flow tangential to the nominal magnetopause expected during all magnetopause crossings and some entries into FTEs. No bipolar flows normal to the nominal magnetopause occur within FTEs unless they rotate and none occur in the magnetosheath unless the boundary waves propagate relative to the magnetosheath flow. IRM satellite observations on October 28, 1984 may be interpreted either in terms of FTEs or boundary waves.

Sibeck, D. G.↗
NASA NTRS1994

A search for upstream pressure pulses associated with flux transfer events: An AMPTE/ISEE case study

On September 19, 1984, the Active Magnetospheric Particle Tracers Explorers (AMPTE) United Kingdom Satellite (UKS) and Ion Release Module (IRM) and International Sun Earth Explorers (ISEE) 1 and 2 spacecraft passed outbound through the dayside magnetopause at about the same time. The AMPTE spacecraft pair crossed first and were in the near-subsolar magnetosheath for more than an hour. Meanwhile the ISEE pair, about 5 R(sub E) to the south, observed flux transfer event (FTE) signatures. We use the AMPTE UKS and IRM plasma and field observations of magnetosheath conditions directly upstream of the subsolar magnetopause to check whether pressure pulses are responsible for the FTE signatures seen at ISEE. Pulses in both the ion thermal pressure and the dynamic pressure are observed in the magnetosheath early on when IRM and UKS are close to the magnetopause, but not later. These large pulses appear to be related to reconnection going on at the magnetopause nearby. AMPTE magnetosheath data far from the magnetopause do not show a pressure pulse correlation with FTEs at ISEE. Moreover, the magnetic pressure and tension effects seen in the ISEE FTEs are much larger than any pressure effects seen in the magnetosheath. A superposed epoch analysis based on small-amplitude peaks in the AMPTE magnetosheath total static pressure (nkT + B(exp 2)/2 mu(sub 0)) hint at some boundary effects, less than 5 nT peak-to-peak variations in the ISEE 1 and 2 B(sub N) signature starting about 1 min after the pressure peak epoch. However, these variations are much smaller than the standard deviations of the B(sub N) field component. Thus the evidence from this case study suggests that upstream magnetosheath pressure pulses do not give rise to FTEs, but may produce very small amplitude signatures in the magnetic field at the magnetopause.

Elphic, R. C.↗
NASA NTRS2007

The Cusp: Open Questions

The northern and southern cusps play an important role in the overall solar wind-magnetosphere- ionosphere interaction. Optical, radar, and ground magnetometer observations of transient events in the dayside auroral oval provide strong evidence for bursty reconnection on the dayside magnetopause occurring along extended reconnection lines during periods of southward and/or ecliptic IMF orientation. During periods of northward IMF orientation, the same ground-based observations provide evidence for reconnection on the magnetopause poleward of the cusps. Cluster observations in the cusp have confirmed the transition in reconnection locations from the dayside to the high-latitude magnetopause as a function of IMF orientation, while in situ observations of the dayside magnetopause indicate that bursty reconnection frequently generates transient events marked by symmetric bipolar magnetic field signatures normal to the nominal magnetopause and magnetic field strength enhancements. In this talk we review recent and invoke new results from analytical, two-dimensional hybrid, and three-dimensional MHD codes to describe the motion and fate of the newly-reconnected magnetic field lines within FTEs, with a view to predicting (I) the locations where events form, (2) the mechanism(s) by which they are generated, (3) their signatures on the dayside and flank magnetopause, (4) their antisunward motion, (5) their interaction with the cusps, and (6) their signatures in the dayside auroral oval, including their occurrence patterns as a function of season. We compare these predictions with observations, in particular the results of a statistical study of Interball-1 FTEs and new multipoint THEMIS observations of FTEs.

Sibeck, D. G.↗
NASA NTRS2009

The Role of Flux Transfer Events in Solar Wind-Magnetosphere Coupling

Recent global MHD simulations of Flux Transfer Events (FTE's)[Dorelli and Bhattachar ee, JGR, 114, 2009] demonstrate that they are topologically complex flux ropes which extend large distances away from the subsolar magnetopause. Thus, FTE's represent a significant perturbation to the magnetopause magnetic field topology. Specifically, prior to the formation of the first FTE of the simulation, the dayside magnetopause has a relatively simple topology consisting of a single magnetic separator draping northward (even for southward IMF) over the dayside magnetopause (in a manner similar to the "overdraping" which occurs under northward IMF conditions). After the first FTE forms, however, the magnetopause becomes topologically more complex, consisting of multiple separators and braided flux domains. One consequence of this topological complexity is that open flux tubes have direct access to the magnetosphere near the subsolar region (such direct access is not possible during steady separator reconnection). Since the magnetopause topology affects the mapping of the solar wind electric field to the magnetosphere, an interesting question arises: What is the effect of FTE generation on the global magnetospheric convection pattern? In this talk, we address this question by exploring the differences between magnetospheric and ionospheric convection before and after the formation of the first FTE of the simulation.

Dorelli, John C.↗
NASA NTRS2012

Deep Solar Activity Minimum 2007-2009: Solar Wind Properties and Major Effects on the Terrestrial Magnetosphere

We discuss the temporal variations and frequency distributions of solar wind and interplanetary magnetic field parameters during the solar minimum of 2007 - 2009 from measurements returned by the IMPACT and PLASTIC instruments on STEREO-A.We find that the density and total field strength were significantly weaker than in the previous minimum. The Alfven Mach number was higher than typical. This reflects the weakness of magnetohydrodynamic (MHD) forces, and has a direct effect on the solar wind-magnetosphere interactions.We then discuss two major aspects that this weak solar activity had on the magnetosphere, using data from Wind and ground-based observations: i) the dayside contribution to the cross-polar cap potential (CPCP), and ii) the shapes of the magnetopause and bow shock. For i) we find a low interplanetary electric field of 1.3+/-0.9 mV/m and a CPCP of 37.3+/-20.2 kV. The auroral activity is closely correlated to the prevalent stream-stream interactions. We suggest that the Alfven wave trains in the fast streams and Kelvin-Helmholtz instability were the predominant agents mediating the transfer of solar wind momentum and energy to the magnetosphere during this three-year period. For ii) we determine 328 magnetopause and 271 bow shock crossings made by Geotail, Cluster 1, and the THEMIS B and C spacecraft during a three-month interval when the daily averages of the magnetic and kinetic energy densities attained their lowest value during the three years under survey.We use the same numerical approach as in Fairfield's empirical model and compare our findings with three magnetopause models. The stand-off distance of the subsolar magnetopause and bow shock were 11.8 R(sub E) and 14.35 R(sub E), respectively. When comparing with Fairfield's classic result, we find that the subsolar magnetosheath is thinner by approx. 1 R(sub E). This is mainly due to the low dynamic pressure which results in a sunward shift of the magnetopause. The magnetopause is more flared than in Fairfield's model. By contrast the bow shock is less flared, and the latter is the result of weaker MHD forces.

solar minimum↗
NASA NTRS1972

Can the ionosphere regulate magnetospheric convection?

Following a southward shift of the interplanetary magnetic field, which implies enhanced reconnection at the nose of the magnetosphere, the magnetopause shrinks from its Chapman-Ferraro equilibrium position. If the convective return of magnetic flux to the magnetopause equalled the reconnection rate, the magnetopause would not shrink. Consequently, there is a delay in the development of magnetospheric convection following the onset of reconnection, which is ascribed to line tying by the polar cusp ionosphere. A simple model relates the dayside magnetopause displacement to the currents feeding the polar cap ionosphere, from which the ionospheric electric field, and consequently, the flux return rate, may be estimated as a function of magnetopause displacement. Flux conservation arguments then permit an estimate of the time scale on which convection increases, which is not inconsistent with that of the substorm growth phase.

Coroniti, F. V.↗
NASA NTRS1984

A dual-satellite study of the spatial properties of FTEs

Reconnection at the earth's dayside magnetopause may manifest itself primarily as a localized and transient process called a flux-transfer event (FTE). The spatial properties of FTEs are investigated directly by examining data from the ISEE satellite pair when the satellites were separated by more than 1000 km in the vicinity of the magnetopause. Examples of magnetosheath and boundary layer FTEs, each having a dimension normal to the magnetopause of order an earth radius, R(E), are shown, and this scale-size result is substantiated statistically for magnetosheath FTEs. When combined with other information, a 1-R(E) normal dimension implies that the voltage associated with the FTE process at one magnetopause location is at least 10 kV. These findings strengthen the view that the magnetic field comprising an FTE is twisted, this twisting appearing to be continuous in sense across the magnetopause and corresponding to a core field-aligned current of magnitude a few hundred kA. Changes in plasma flow speed and direction are found to be associated with FTEs. The transverse field and flow perturbations accompanying the three magnetosheath FTEs studied here satisfy approximately the Walen relation, the relation which describes a propagating Alfven wave.

Saunders, M. A.↗
NASA NTRS1989

Flux transfer events: Reconnection without separators

A topological analysis of a simple model magnetic field of a perturbation at the magnetopause modeling an apparent flux transfer event is presented. It is shown that a localized perturbation at the magnetopause can in principle open a closed magnetosphere by establishing magnetic connections across the magnetopause. Although the model field exhibits neutral points, these are not involved in the magnetic connection of the flux tubes. The topological substructure of a localized perturbation is analyzed in a simpler configuration. The presence of both signs of the magnetic field component normal to the magnetopause leads to a linkage of topologically different flux tubes, described as a flux knot, and a filamentary substructure of field lines of different topological types which becomes increasingly complicated for decreasing magnetic shear at the magnetopause.

Hesse, M.↗
NASA NTRS1994

Double vision: Remote sensing of a flux transfer event with ISEE 1 and 2

A flux transfer event recorded by the International Sun-Earth Explorers (ISEE) 1 and 2 spacecraft in the northern hemisphere near local noon is examined using analysis techniques developed recently for determination from single satellite magnetometer data of the orientation and cross-sectional shape of a two-dimensional disturbance moving along the magnetopause. The analysis is performed independently on the magnetic field data recorded by each satellite, enabling us to check the consistency of the results and thus to benchmark the method. The analysis is also extended to utilize the dual magnetometer measurements for determination of the event speed and size (which for a single spacecraft requires use of three-dimensional plasma velocity data to obtain a deHoffmann-Teller frame velocity) and for accommodating acceleration of the magnetopause normal to itself, which results in a curved spacecraft trajectory in the frame of reference moving with the disturbance. The analysis of the disturbance reveals that while its overall size is about 34,000 km in a direction parallel to the magnetopause but perpendicular to the event axis, the actual size of the bulge causing the field disturbance may have been as small as 9000 km with a dimension perpendicular to the magnetopause of the order of 1200 km. The bulge is found to be traveling at a speed of about 140 km/s toward the northern dawn quadrant of the dayside magnetopause. While plasma data from ISEE 2 provides evidence of plasma jetting, which suggests the event may be associated with reconnection, the orientation of the flux tube axis obtained from analysis is significantly different from the expected reconnection line direction. A discussion of possible interpretations of this bulge orientation and motion is provided.

Walthour, D. W.↗