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At least 37 records · Page 2
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NASA NTRS2004

Average Characteristics of Triggered and Nontriggered Substorms

Magnetic field data from ground stations, geosynchronous orbit, and magnetotail are examined to study the response to substorm activity with and without apparent interplanetary magnetic field (IMF) perturbations. Global substorms are identified using a sudden, persistent decrease in the AL index. The onset of this global expansion is taken to be the time of the Pi2 burst nearest to the beginning of the AL decrease. IMF triggers were identified subjectively through visual scanning of the data. Both northward turnings of the IMF B, and decreases in the amplitude of the By component were considered as possible triggers. Two different solar wind monitors were used in the investigation: IMP 8 in a circular orbit with a distance between approx.12 and approx.35 R(sub E) from the Earth-Sun line and ISEE 2 in an elliptical orbit with a distance of only approx.5- 10 R(sub E) from the Earth-Sun line. The results of superposed epoch analysis show that the temporal response from ground stations, geosynchronous orbit, and magnetotail are nearly identical for triggered (with apparent IMF perturbation) and nontriggered (without apparent IMF perturbation) substorms. It is therefore concluded that the nontriggered substorms are not a different form of activity than triggered substorms. However, we demonstrate that the magnitude of the response is different for the two types of substo&. By every measure considered, triggered substorm are systematically larger than nontriggered substorms. We interpret the fact that nearly 40% of all substorms cannot be associated with an IMF trigger as evidence that substorms are caused by an internal instability. However, the fact that so many appear to be triggered suggests that this internal instability is susceptible to external perturbations by the IMF. The fact that triggered substorms are larger than nontriggered substorms is counterintuitive, and we have no explanation for the observation.

Hsu, Tung-Shin↗
NASA NTRS1978

The use of ground magnetograms to time the onset of magnetospheric substorms

A critical review of magnetospheric substorm timing techniques is presented. Attention is given to substorm timing with auroral zone magnetograms, substorm onset timing with midlatitude magnetograms, multiple onset substorms, and a phenomenological model for multiple onset substorms. Emphasis is placed on some pitfalls that have led some investigators to err in substorm onset determinations. In particular, multiple onset substorms appear to account for many of the reported discrepancies in substorm timing. It is suggested that present idealized models of substorm morphology must be modified to include the multiple onset substorms.

Mcpherron, R. L.↗
NASA NTRS1995

Observations in the vicinity of substorm onset: Implications for the substrom process

Multi-instrument data sets from the ground and satellites at both low and high altitude have provided new results concerning substorm onset and its source region in the magnetosphere. Twenty-six out of 37 substorm onset events showed evidence of azimuthally spaced auroral forms (AAFs) prior to the explosive poleward motion associated with optical substorm onset. AAFs can span 8 hours of local time prior to onset and generally propagate eastward in the morning sector. Onset itself is, however, more localized spanning only about 1 hour local time. AAF onset occur during time periods when the solar wind pressure is relatively high. AAFs brighten in conjunction with substorm onset leading to the conclusion that they are a growth phase activity casually related to substorm onset. Precursor activity associated with these AAFs is also seen near geosynchronous orbit altitude and examples show the relationship between the various instrumental definitions of substorm onset. The implied mode number (30 to 135) derived from this work is inconsistent with cavity mode resonances but is consistent with a modified flute/ballooning instability which requires azimuthal pressure gradients. The extended source region and the distance to the open-closed field line region constrain reconnection theory and local mechanisms for substorm onset. It is demonstrated that multiple onset substorms can exist for which localized dipolarizations and the Pi 2 occur simultaneously with tail stretching existing elsewhere. These pseudobreakups can be initiated by auroral streamers which originate at the most poleward set of arc systems and drift to the more equatorward main UV oval. Observations are presented of these AAFs in conjunction with low- and high-altitutde particle and magnetic field data. These place the activations at the interface between dipolar and taillike field lines probably near the peak in the cross-tail current. These onsets are put in the context of a new scenario for substorm morphology which employs individual modules which operate independently or couple together. This allows particular substorm events to be more accurately described and investigated.

Elphinstone, R. D.↗
NASA NTRS1998

On the Predictability of Substorms Following Sharp Northward Turnings of the IMF

It has been shown that there is an association between changes of the interplanetary magnetic field (IMF) that are expected to lead to a reduction in magnetospheric convection (northward turnings, reductions) and the onset of the expansion phase of substorms. This has been previously demonstrated by analyses of IMF data during time intervals associated with identified substorm onsets. Here we examine whether observations of northward turnings of the IMF can be used to predict the occurrence of substorms. We first identified sharp northward turnings that follow an interval of steady, southward IMF using measurements from the Wind spacecraft during the first 180 days of 1997. We also required that the northward turning be observed by either IMP-8 or GEOTAIL, in addition to Wind, and that one of the observing satellites be sufficiently close to the Earth-Sun line, or that the two observing satellites be sufficiently separated, that we are reasonably certain that the northward turning affected the magnetosphere. We also used the dual observations to estimate the arrival of the northward turning at the Earth. Using these criteria, we predicted 17 substorms. We then searched for the following signatures of substorm onset around the time of the predicted onset: auroral brightening followed by auroral bulge expansion observed by Polar UVI, geosynchronous particle injection, geosynchronous magnetic field dipolarization, and an appropriate magnetic disturbance at the surface of the Earth. Of the 17 predictions of substorms, 10 were successful in that a substorm onset was observed within 12 min of the predicted onset, 1 is indeterminate due to a lack of data at the Earth, 1 had unusual activity that we have not been able to identify, and 5 were unsuccessful. The failure of these last 5 predictions is explicable. Two of the northward turnings that failed to produce substorms were preceded by the lowest average of the set. The remaining 3 were the only cases in which the northward turning was accompanied by a simultaneous sharp increase. The increase would be expected to offset the decrease in convection that would otherwise be expected to be associated with a northward turning. These results indicate that it is an IMF change that leads to a reduction in convection, rather than just a northward turning or reduction that is associated with substorms, and that at least some substorms can be predicted by measurements of the IMF.

Blanchard, G. T.↗
NASA NTRS1998

A Substorm Triggered by a Sudden Drop in Dynamic Pressure

In traditional substorm theories the growth phase is driven by an extended period of southward IMF, which transfers large amounts of re-connected magnetic flux into the magnetotail. The subsequent substorm onset may be triggered by a northward turning of the IMF. We investigate the possibility that variations in dynamic pressure may produce these same effects for some substorms lacking a clear IMF Bz signature for both the growth phase and the onset. A sustained increase in dynamic pressure may raise the energy stored in the compressed magnetosphere system (similar to the growth phase), while a subsequent decrease in dynamic pressure may allow some of this stored energy to be released, possibly triggering a substorm. We present a global magnetospheric simulation of such a substorm, which was also imaged by the Ultraviolet Imager (UVI) on the Polar spacecraft. The plasma density rapidly doubled about 90 minutes before the substorm onset with relatively little change in the solar wind velocity and the IMF during this period. Likewise, a sudden decrease in the density almost back to the original levels occurred close to the time of substorm onset, again with no significant change in solar wind velocity or IMF, and thus this dynamic pressure drop may have triggered the substorm. We investigate whether the resulting dynamic pressure increase in the simulation produces a growth phase, a feature that is very successfully modeled by global magnetospheric simulations. We also search for the signature of substorm onset in diagnostics such as the field-aligned currents and the position of the neutral line, if any, in the magnetotail.

Elsen, R.↗
NASA NTRS1999

Predictions of Substorms and Intensifications Following Northward Turnings of the IMF

Substorms are often observed to occur at the end of intervals of Southward interplanetary magnetic field (IMF), i.e. following the northward turning. Despite the significant correlation between northward turning and substorms, no direct causal relationship between northward turnings and substorms has been demonstrated. Assuming such a causal relationship, we predict that substorms will occur within a particular interval following the observation of a northward turning in the IMF. We observe 16 northward turnings following steady, southward IMF in data taken by the WIND spacecraft magnetic field instrument (MFI). To ensure that the northward turning was observed at the magnetosphere, we require that the northward turning also be observed by instruments on either one of Geotail or IMP-8 while the separation of the second spacecraft from WIND was more that 10 R(sub E). These two-spacecraft observations also allow us to predict more accurately the arrival time of the northward turning at the Earth. Of the predictions substorms, 10 predictions were clearly successful to within +/- 12 min. Five predictions failed, but the failures reveal clear shortcomings in the criteria for a northward turning that we correct. The failures were caused by an increase in the absolute value of B(sub YGSM) simultaneous with the northward turning in 3 cases, and a weak southward IMF preceding the northward turning in 2 cases. The final northward turning arrived in the recovery phase of an ongoing substorm, and resulted in unusual auroral activity. The implication of the predictability of substorms following sharp northward turnings is that the postulated causal relationship between northward turnings and substorm onset exists. The effect of increases in the absolute value of B(sub YGSM) to negate the triggering ability of northward turnings suggests that the triggering mechanism involves sharp reductions in the magnetospheric convection electric field.

Blanchard, G. T.↗
NASA NTRS2009

Hemispheric Asymmetries in Substorm Recovery Time Scales

Previous statistical observations have shown that the recovery time scales of substorms occurring in the winter and near equinox (when the nighttime auroral zone was in darkness) are roughly twice as long as the recovery time scales for substorms occurring in the summer (when the nighttime auroral region was sunlit). This suggests that auroral substorms in the northern and southern hemispheres develop asymmetrically during solstice conditions with substorms lasting longer in the winter (dark) hemisphere than in the summer (sunlit) hemisphere. Additionally, this implies that more energy is deposited by electron precipitation in the winter hemisphere than in the summer one during substorms. This result, coupled with previous observations that have shown that auroral activity is more common when the ionosphere is in darkness and is suppressed when the ionosphere is in daylight, strongly suggests that the ionospheric conductivity plays an important role governing how magnetospheric energy is transferred to the ionosphere during substorms. Therefore, the ionosphere itself may dictate how much energy it will accept from the magnetosphere during substorms rather than this being an externally imposed quantity. Here, we extend our earlier work by statistically analyzing the recovery time scales for a large number of substorms observed in the conjugate hemispheres simultaneously by two orbiting global auroral imagers: Polar UVI and IMAGE FUV. Our current results are consistent with previous observations. The recovery time scales are observed to be longer in the winter (dark) hemisphere while the auroral activity has a shorter duration in the summer (sunlit) hemisphere. This leads to an asymmetric energy input from the magnetosphere to the ionosphere with more energy being deposited in the winter hemisphere than in the summer hemisphere.

Fillingim, M. O.↗
NASA NTRS1987

Simultaneous observations of the near-earth and distant geomagnetic tail during a substorm by ISEE-1, ISEE-3 and geostationary spacecraft

The structure of the geomagnetic tail during a substorm is investigated by combining plasma, magnetic field, and energetic particle data from the ISEE-3 spacecraft in the deep tail with similar near-earth observations from ISEE-1 and geostationary spacecraft. The observations can be interpreted in terms of the neutral-line model of substorms and indicate the formation of a closed-loop field region (plasmoid) following substorm onset, which is ejected down the tail. The plasmoid is observed to have a double-loop field strucure. This may be the result of a second substorm onset occurring about 25 min after the first, producing a further near-earth neutral line and closed field loop. During the substorm recovery phase, the substorm neutral line moves tailward to beyond 130 earth radii from earth by some 3 h after substorm onset.

Richardson, I. G.↗
NASA NTRS1992

ISEE 3 plasmoid and TCR observations during an extended interval of substorm activity

On April 9-11, 1983, the ISEE 3 spacecraft was continuously located within the earth's magnetotail for more than 36 hours at downstream distances of X = -76 to -80 R(e). During this span of time, 12 major intervals of substorm activity were observed in the AL index with good ISEE 3 telemetry coverage for 11 of them. In addition, there were two small substorms outside of these intervals, both with complete observations in the distant tail. This unusual ISEE 3 data set provides a unique opportunity to test the predictions of the near-earth neutral line model. In particular, the hypothesis that energy stored in the tail lobes during the growth phase is later dissipated, in part, through the release of one or more plasmoids following expansion phase onset is examined. Clear growth phase enhancements in the lobe magnetic field intensity preceded the onsets of nine of the substorms. Plasmoids, or their lobe signatures, traveling compression regions (TCRs), were observed at ISEE 3 in association with all 11 of the major substorm intervals for which there were ISEE observations, as well as for the two small substorms. No plasmoids or TCRs were observed in the absence of substorm activity. If these ISEE 3 observations are representative, then the release of plasmoids down the tail may be a feature common to all substorms.

Slavin, J. A.↗
NASA NTRS1993

Geomagnetic substorm association of plasmoids

The relationship of geomagnetic substorms and plasmoids is examined by determining the correlation of the 366 plasmoids identified by Moldwin and Hughes (1992) with ground auroral zone magnetograms and geosynchronous particle data signatures of substorm onsets. Over 84 percent of the plasmoid events occurred between 5 and 60 min after the substorm onsets. We also find near one-to-one correlation between large isolated substorm signatures in the near-earth region and signatures consistent with a passing plasmoid in the distant tail. However, there does not appear to be an absolute correspondence of every substorm onset to a plasmoid signature in the deep tail especially for periods of prolonged disturbance that have multiple substorm onsets. A correlation of interplanetary magnetic field Bz south with plasmoid observations was also found. The locations of the near- and far-earth reconnection sites are estimated using the time of flight of the plasmoids from substorm onset to their observation at ISEE 3. The estimates of the near- and far-earth reconnection sites are highly variable and range from 10 to 140 R(E).

Moldwin, Mark B.↗
NASA NTRS1993

A statistical relationship between the geosynchronous magnetic field and substorm electrojet magnitude

The relationship between the geosynchronous magnetic field variations during substorms measured by GOES 5 and the auroral electroject as measured by AE and Poste de la Baleine is examined. It is found that the more taillike the field prior to the local onset, the greater the dipolarization of the field during the substorm. The greater the deviation of the field from a dipolar configuration, the larger the change in AE during the event. It is inferred that stronger cross-tail currents prior to the substorm are associated with larger substorm-associated westward electrojets and thus more intense substorms. Since the westward electroject is the ionospheric leg of the substorm current wedge, it is inferred that the substorm-associated westward electrojet is drawn from the near-earth region. Most of the current diversion is found to occur in the near-earth magnetotail.

Lopez, Ramon E.↗
NASA NTRS1996

Occurrency frequency of substorm field and plasma signatures observed near-earth by ISEE-1/2

The onset of the majority of substorms occurs when the tail field stops growing more tail-like and begins to become more dipolar. This corresponds to the onset signatures on the ground and in geosynchronous orbit. The AE indices and the IGS Pi 2 data were used to determine the major substorm onsets of 1978 and 1979. The time delay between successive substorms, the distribution of the substorm growth phase duration and the probability of tailward flows were determined as a function of spacecraft location. About a half of the substorms exhibit a plasma signature including earthward or tailward flows or plasma sheet drop out and recovery. Earthward flows are often seen at substorm onset, and almost always during substorm recovery. Tailward flows are occasionally seen at onset as the spacecraft is close enough to the neutral sheet. The experimental results are compared to predictions based on the neutral line and current sheet disruption models.

Hsu, T. S.↗
NASA NTRS1998

Magnetospheric Substorms and Tail Dynamics

This grant funded several studies of magnetospheric substorms and their effect on the dynamics of the earth's geomagnetic tail. We completed an extensive study of plasmoids, plasma/magnetic field structures that travel rapidly down the tail, using data from the ISEE 3 and IMP 8 spacecraft. This study formed the PhD thesis of Mark Moldwin. We found that magnetically plasmoids are better described as flux-ropes (twisted magnetic flux tubes) rather than plasma bubbles, as had been generally regarded up to that point (Moldwin and Hughes, 1990; 1991). We published several examples of plasmoids observed first in the near tail by IMP 8 and later in the distant tail by ISEE 3, confirming their velocities down tail. We showed how the passage of plasmoids distorts the plasma sheet. We completed the first extensive statistical survey of plasmoids that showed how plasmoids evolve as they move down tail from their formation around 30 RE to ISEE 3 apogee at 240 RE. We established a one-to-one correspondence between the observation of plasmoids in the distant tail and substorm onsets at earth or in the near tail. And we showed that there is a class of plasmoid-like structures that move slowly earthward, especially following weak substorms during northward IMF. Collectively this work constituted the most extensive study of plasmoids prior to the work that has now been done with the GEOTAIL spacecraft. Following our work on plasmoids, we turned our attention to signatures of substorm onset observed in the inner magnetosphere near geosynchronous orbit, especially signatures observed by the CRRES satellite. Using data from the magnetometer, electric field probe, plasma wave instrument, and low energy plasma instrument on CRRES we were able to better document substorm onsets in the inner magnetosphere than had been possible previously. Detailed calculation of the Poynting flux showed energy exchange between the magnetosphere and ionosphere, and a short burst of tailward convective flow just prior to onset, suggesting the active role of the ionosphere in the onset process, and adding credibility to the ballooning instability theory of substorm onset. This grant also supported a number of other substorm studies and reviews. These are represented by the list of publications and meeting presentations resulting out of this grant.

Hughes, W. Jeffrey↗
NASA NTRS1999

Dynamics of the Auroral Luminosity Boundary of the Polar Cap During Substorms

The area of the polar cap during substorms has been measured using images from the Polar Ultraviolet Imager (UVI) for different interplanetary magnetic field (IMF) conditions. Changes in the poleward boundary of auroral luminosity have been analyzed in relation to substorm phase and IMF orientation. Reconnection models of flux transport into the polar cap during the substorm growth phase, and loss from the polar cap during the expansion phase, provide a framework by which these UVI observations can be analyzed. By comparison of the observations with the model predictions we can determine to what extent these models accurately predict the polar cap dynamics, and also where anomalous behavior calls for a new understanding of the dynamics beyond what these models provide. It was found that the polar cap boundary near noon and midnight usually shifted down in latitude by 1-2 degrees and 3-4 degrees respectively, increasing the area of the polar cap during the substorm growth phase as predicted. However, this growth phase phenomenon also unexpectedly occurs independently of the IMF Bz component, as shown for a substorm on January 9, 1997. The polar cap area also increased due to motion of the dawn and dusk aurora to lower latitudes, although the latitudinal shifts were asymmetric, not always concurrent, and continued well into the substorm expansion phase. The polar cap area decreased immediately following the expansion phase due to the poleward motion of the aurora on the nightside, consistent with the model prediction. What is not explained by the models is that the poleward auroral boundary in the nightside region sometimes reached very high latitudes (greater than 80 degrees MLat) greatly decreasing the polar cap area, independent of the magnitude of the substorm.

Brittnacher, M. J.↗
NASA NTRS2016

Special Issue the 12th International Conference on Substorms

The 12th International Conference on Substorms (ICS-12) was held at the Ise-Shima Royal Hotel in Shima, Japan, on November 10-14, 2014. There were 125 attendees including 68 from foreign countries. The ICS has been held every 2 years since 1992 to discuss substorms, which are fundamental global-scale disturbances in the Earth's magnetosphere. The year 2014 marked the 50th anniversary of the first publication about substorms (Akasofu 1964). The conference included three tutorial lecturers (Profs. S.-I. Akasofu, V. Angelopoulous, and D. Baker), as well as many international scientists, to discuss substorm processes in the tail, their Interactions with the inner magnetosphere and the ionosphere, substorm currents and their dynamics and energetics, the role of MagnetoHydroDynamics (MHD) and kinetic instabilities, storm-substorm relationships, ULFELFVLF waves, and non-Earth substorm-like features. Prof. Akasofu also gave an evening talk about the history of auroral research since the nineteenth century with photographs that inspired and intrigued the young scientists and students in attendance.

Shiokawa, Kazuo↗
NASA NTRS1973

Substorms in space - The correlation between ground and satellite observations of the magnetic field

Several of the events criticized by Akasofu (1972) are reexamined. It is concluded that there is no simple one-to-one relationship between polar magnetic substorms and magnetospheric substorms as defined by midlatitude magnetograms. It appears that in some cases polar magnetic substorms occur during the growth phase of a magnetospheric substorm. Magnetospheric observations are more systematically organized by midlatitude onsets than by auroral zone onsets. The determination of onset times is discussed together with the determination of substorm similarity, the phenomenological model of magnetic variations during magnetospheric substorms, the event of February 25, 1967, and the complex event of February 13, 1968.

Mcpherron, R. L.↗
NASA NTRS1974

Study of a small magnetospheric substorm

Data from a rocket launched into the expansion phase of an auroral substorm have been compared with data from numerous ground stations and several space vehicles. It is shown that this magnetic and auroral substorm has most of the features of larger substorms; thus it is implied that the same plasma processes are involved in all substorms. Some evidence is presented to link the auroral breakup with the field lines conjugate to the inner edge of the plasma sheet. The implications of these two results for substorm morphology and the triggering mechanism of the substorm instability are discussed. It is concluded that spatial gradients in plasma temperature are a likely cause.

Johnstone, A. D.↗
NASA NTRS1980

Magnetic variations during substorms

Extensive observations of magnetic field variations during auroral substorms have been used to construct phenomenological models of the substorm process. In one model, three substorm phases, growth, expansion and recovery correspond respectively to energy storage, energy release and return to equilibrium. New data and more detailed studies show the simple model required modification. Consideration is given to a number of recent studies of substorm magnetic field variations which illustrate the limitations of the simple model. These include the existence of several substorm expansions following a single growth phase; the frequent occurrence of multiple onsets of a single substorm; plasma sheet thinning prior to each onset; correlations between northward fluctuations of the IMF and expansion onset. These observations are incorporated into a revised phenomenological model.

Mcpherron, R. L.↗