Bipolar Signature Research Articles

Improved atmospheric understanding through exploratory data analysis and complementary modeling: The urban KPbC system

Abstract Univariate and multivariate exploration of urban carbonaceous aerosol data have revealed some interesting and important facets of aerosol carbon characteristics, sources and modeling assumptions. Data were obtained from wintertime sampling in Albuquerque—a time and location for which the two-source (motor vehicle, woodburning) aerosol carbon hypothesis is believed valid. Carbon isotopic tracers (14C,12C) provided a unique, absolute and direct means for tracing and quantifying fossil carbon and wood carbon contributions to the total aerosol carbon. Lead and mineral-corrected potassium served as unique, but indirect and uncalibrated (non-absolute) tracers for the two carbonaceous aerosol sources. Results for the two models showed excellent consistency for daytime carbon and good agreement for most samples, but the indirect tracer model underpredicted woodcarbon aerosol at night. Multivariate data exploration, including temperature as well as both elemental and organic (polycyclic) tracers suggested that the regression lack of fit, and excess nocturnal wood carbon were due to a third “statistical” component in the data, which in turn reflects a chemical non-linearity, apparently associated with volatility. Important conclusions from analysis of the combined isotopic, elemental and organic data from this study are that: (1) high molecular weight, condensed polycyclic hydrocarbons [benzo (ghi) perylene and coronene] appeared to be effective tracers for motor-vehicle carbon; and (2) at least two independent aerosol carbon components derived from woodburning, one of which—presumably involving volatile-to-aerosol carbon condensation—was uncoupled from elemental (K) tracer emissions from woodburning. Finally, some insight was gained into the process of characterizing urban and regional aerosol “fingerprints” on the basis of the observed variance-covariance structure of their isotopic and elemental composition. This was illustrated for the wintertime Albuquerque atmosphere, which was found to exhibit a bi-polar signature.

Observations of earthward and tailward propagating flux rope plasmoids: Expanding the plasmoid model of geomagnetic substorms

A survey of IMP 8 magnetometer data for plasmoid signatures during magnetospheric intervals from 1981 through 1983 found 16 plasmoids and 37 traveling compression regions as well as two earthward propagating flux ropes and 19 south‐north bipolar lobe signatures. The properties of these relatively near‐Earth plasmoids, traveling compression regions, and earthward propagating flux ropes and a qualitative model for their formation are presented. The plasmoids have estimated sizes, durations, magnetic field signatures, downtail velocities, and substorm associations very similar to those of the plasmoids identified in ISEE 3 deep‐tail observations. The occurrence frequency of these near‐Earth plasma sheet plasmoids is significantly smaller than that of plasmoids found in the mid‐ and deep tail with ISEE 3. The earthward propagating flux ropes are characterized by a south‐north bipolar turning in the GSM Bz component, are localized near the noon‐midnight meridional plane, and are strongly correlated with interplanetary magnetic field Bz north and small isolated high latitude geomagnetic substorms. These events are also apparently very rare and/or spatially localized. We propose that these structures are “proto‐plasmoids,” i.e., plasmoids for which near‐Earth magnetic reconnection stopped before all the closed plasma sheet field lines were reconnected. The proto‐plasmoids are then “trapped” inside closed magnetic field lines and propagate earthward owing to the effect of the distant X‐line's earthward plasma flow. We suggest that the two different “types” of plasmoids are due to the different energy states of the magnetosphere during periods of southward and northward interplanetary magnetic field.

High‐time resolution measurements of upstream magnetic field and plasma conditions during flux transfer events at the Earth's dayside magnetopause

We present preliminary results of a study of upstream magnetic field and plasma conditions measured by IRM during flux transfer events observed at the Earth's magnetopause by CCE. This study was designed to determine the importance of various upstream factors in the formation of bipolar magnetic field signatures called flux transfer events (FTEs). Six FTE encounters were examined. Three cases were when the two satellites were on similar magnetic field lines. Preliminary investigation showed that fluctuations occurred in the Bz component of the interplanetary magnetic field (IMF) resulting in a southward field preceding the FTE in all three of these cases. In two of these cases, the changes were characterized by a distinct rotation from a strong southward to a strong northward field. There were also accompanying changes in the dynamic and thermal pressure in the solar wind immediately before the FTE was encountered. Examination of the 3d plasma distributions showed that these pulses were due to the addition of energetic upstreaming foreshock particles, There were no consistent changes in either Bz or the plasma pressure at IRM for the three events when the satellites were not connected by the IMF.

The Galileo Earth encounter: Magnetometer and allied measurements

The Galileo spacecraft flew by Earth on December 8, 1990, at high speed along a trajectory that traversed the magnetotail and the near Earth magnetosphere. Galileo's orbit through a region of the magnetotail from which limited data are available provided a unique opportunity to study a number of substorm‐related phenomena. Several groups cooperated in collecting correlative data in order to take advantage of this special opportunity. Fortunately, geomagnetic conditions were rather disturbed during the entire day, and an interplanetary shock passed Earth when the spacecraft was in the magnetotail at about 30 RE geocentric distance. In this first report we provide an overview of the Galileo magnetometer observations from the crossing of the tail magnetopause at an antisolar distance of close to 100 RE through exit into the solar wind on the dayside. We link these measurements with correlative data from ground stations and from IMP 8 which was ideally located to serve as a monitor of the solar wind upstream of the bow shock. Based on our analysis, we present a time line of the important geomagnetic events of the day that we believe provides a framework for the full multi‐instrument analysis of the flyby data. In this paper we use the observations to investigate aspects of the relationship between magnetotail dynamics and the separate intensifications of a multiple onset substorm inferred from ground‐based data. The spacecraft spent 6 h downstream of lunar orbit, of which more than 4 h were spent outside of the plasma sheet in regions where traveling compressional regions (TCRs) should have been apparent. Although six substorm intensifications were recorded on the ground during this interval, we did not observe a detectable TCR or plasmoid for every intensification. Our interpretation has important implications for the description of substorm dynamics in the tail. We propose that the signatures associated with individual substorm intensifications are localized in dawn‐to‐dusk extent even at remote locations in the magnetotail, just as they are in the ionosphere, and that the tail disturbances associated with successive substorm intensifications step across the tail towards the dusk flank. This latter interpretation is appealing as it can explain the failure of Galileo to observe a signature associated with each intensification without invalidating the conclusion of ISEE 3 investigators that in the same region of the magnetotail at least one signature can be associated with each substorm viewed as a collection of individual intensifications. Plasmoidlike signatures with strong axial fields along the GSM y axis and parallel to the By of the interplanetary magnetic field (IMF) were present when the spacecraft was embedded close to the center of the plasma sheet. We interpret these signatures as flux ropes, that is, twisted magnetic structures with one end possibly tied to the ionosphere. The modeled structure yields j × B/jB ≪ 1 which suggests that the flux ropes are magnetically force free to within the limitations of the model. We point out that plasmoids and flux ropes form a continuum of structures distinguished by the magnitude of By. Our observations lend additional support to the view that bipolar Bz signatures in the magnetotail may often be better described as flux ropes than as disconnected plasmoids. Our other principal results are only summarized in this paper; they will be discussed in greater detail elsewhere. They include (1) additional evidence that the IMF By controls the lobe magnetic field only in the quadrants that are magnetically linked to the solar wind, and (2) evidence that the low‐frequency response (the classic “sudden impulse” or SI signature) to a solar wind shock can be absent in the magnetic signature obtained within a high β plasma sheet. We believe that these observations will provide insight useful for improving phenomenological models of substorms.

On the formation and evolution of plasmoids: A survey of ISEE 3 geotail data

ISEE 3 magnetometer and electron plasma measurements from the 1983 Geotail Mission were surveyed to determine the magnetic and plasma properties of plasmoids and their evolution with distance downtail. Events were selected on the basis of a bipolar magnetic signature in either the geocentric solar magnetospheric Bz and/or By component; most had Bz bipolar signatures. We found 366 events consistent with this signature while ISEE 3 was in the plasma sheet. ISEE 3 observed plasmoids all along its trajectory whenever it was in the plasma sheet. Plasmoids are characterized by high‐speed plasma flow. Plasmoid length was determined using both the magnetometer and the electron plasma velocity data. We found the average length of plasmoids is 16.7 ± 13.0 RE, significantly smaller than previous estimates. Many plasmoids have a well‐defined magnetic core field, characterized by a field strength maximum at the center of the pass through the structure. Plasmoids appear to be relatively stable structures once their formation process is complete. The size, velocity, magnetic core strength, and Bz field amplitude of plasmoids do not depend on distance beyond 100 RE downtail. The average electron temperature inside plasmoids drops by a factor of 2 and the electron density increases by a factor of 2 as plasmoids propagate from near Earth distances (within 100 RE of the Earth) to the deep tail. We conclude that the stable size of the plasmoids, the density increase and the temperature decrease are consistent with a flux of cold electrons into the plasmoid. The strong correlation of interplanetary magnetic field By an hour before the event with the strength and direction of By observed inside plasmoids, the existence of events with the bipolar signature in both the By and Bz components, and the possible mass flux all are consistent with plasmoids being “open” magnetic structures.

Gravity field and deep structure of the Bengal Fan and its surrounding continental margins, northeast Indian Ocean

A revised gravity anomaly map for the northeast Indian Ocean shows that the shelf edge underlying the eastern continental margin of India is a rather narrow but extensively linear gravity low (minimum free-air = −149 mGal). The Bengal Fan seaward of the shelf has a depressed gravity field (average free-air = −20 to −30 mGal) in spite of the enormous thickness of sediments of as much as 10–15 km. The two buried ridges below the Bengal Fan—the 85° East and 90° East Ridges—have a large negative (−75 mgal) and a substantial positive (40 mGal) free-air anomaly, respectively. The Andaman and Burmese arcs lying along the east margin of the Bengal Fan are active subduction areas which have typical bipolar gravity signatures with a maximum amplitude of 300 mGal. Gravity interpretation for three regional traverses across the central and northern parts of the Bengal Fan and their surrounding continental margins suggests that a thickened oceanic crustal wedge juxtaposes the transitional crust under the eastern continental slope of India; the 85° East Ridge, that was created when the Indian Ocean lithosphere was very juvenile, appears to underlie a nearly 10 km thick and 120 km wide oceanic crustal block consisting of the ridge material embedded in the upper lithosphere; while the 90° East Ridge submarine topography/buried load below the Bengal Fan is probably isostatically compensated by a low-density mass acting as a cushion at the base of the crust. The Bengal Fan crust, with its thick sediment layer, is carried down the Andaman subduction zone to a depth of about 27 km where, possibly, phase transition takes place under higher pressure. The maximum sediment thickness at the Andaman-Burmese subduction zone is of the order of 10–12 km. The gravity model predicts a low density zone about 60 km wide below the Andaman-Burmese volcanic arc, penetrating from crustal to subcrustal depths in the overriding Burma plate. A more complex density distribution is however, envisaged for the Andaman volcanic arc that is split by the Neogene back arc spreading ridge. The ocean-continent crustal transition possibly occurs farther east of the volcanic arc; below the Shan plateau margin in Burma or below the Mergui terrace at the Malayan continental margin east of the Andaman Sea.

Different FTE signatures generated by the bursty single X line reconnection and the multiple X line reconnection at the dayside magnetopause

Magnetic signatures associated with the time‐dependent magnetic reconnection processes at the dayside magnetopause are studied based on two‐dimensional compressible MHD simulations. In the simulations, magnetic and plasma signatures resemblant to the observed flux transfer events (FTEs) can be generated either by the magnetic bulges formed during the bursty single X line reconnection (BSXR) or by the magnetic islands (flux tubes) formed during the multiple X line reconnection (MXR). It is found that the FTE magnetic signatures are not exhibited on the magnetospheric side if the FTEs are due to the BSXR process and Bm/Bs ≥ 1.7, where Bm and Bs are the magnetic field strength in the magnetosheath and in the magnetosphere, respectively. On the other hand, the bipolar FTE signatures can be detected on both the magnetosphere and magnetosheath sides if the FTEs are caused by the MXR process and Bm/Bs ≤ 2.6. When Bm/Bs > 2.6, the bipolar FTE signatures in the magnetosphere site become too small to be detected even if magnetic islands are formed during the MXR process. Furthermore, for Bm/Bs > 1, the region for the detection of FTE signatures in the magnetospheric side is smaller than that in the magnetosheath side. Since at the dayside magnetopause the typical value of Bm/Bs is 1–3, the simulation results indicate that more FTE signatures can be detected in the magnetosheath side than in the magnetosphere. It is also found that the MXR process often generates a clear bipolar Bn signature while the BSXR process tends to produce FTEs with a monopolar Bn signature near the reconnection region and a highly asymmetric bipolar Bn signature away from the reconnection region.

3D resistive MHD computations of magnetospheric physics

Numerical modeling of magnetospheres involves a number of aspects of magnetospheric physics, which may roughly be divided into global, local and microscopic dynamics. Global dynamics should include the interaction of the magnetosphere with the stellar wind plasma and of different parts of the magnetosphere with each other. By local dynamics we address the physics of a certain part of the magnetosphere with a reduced number of length and time scales. Examples are the plasma and energy transport at the magnetopause or in the magnetotail. Important transport coefficients for global and local processes are determined by the microscopic plasma dynamics. Clearly a selfconsistent modelling including all of these aspects is far beyond today's computer facilities. Even the rather different length and time scales in modeling one of the mentioned aspects of magnetospheric physics requires careful consideration.In this paper we will concentrate on the local dynamics of the magnetopause with special emphasis on the formation of flux transfer events (FTE'). FTE's seem to contribute a significant if not the dominant part of the plasma and energy transport from the solar wind into the earth's magnetosphere. We will summarize the conditions for modeling these phenomena due to different length scales, plasma parameters and relevant instabilities. The results of a three-dimensional resistive MHD code will be presented, which has been developed for this application. This code offers the option of using the Lax-Wendroff or the leapfrog integration scheme, where large efficiency has been achieved by a high degree of vectorization and the use of a nonuniform grid for three Cartesian directions.The results obtained by this MHD code recover typical observational features of FTE's, e.g. the bipolar signature of the magnetic field component normal to the magnetopause. Some conclusions concerning the observations of FTE's will be drawn. Furthermore we will compare these results with theoretical considerations.

Simultaneous measurements of the magnetopause and flux transfer events at widely separated sites by AMPTE UKS and ISEE 1 and 2

On September 19, 1984, the ISEE 1 and 2 and AMPTE UKS and IRM spacecraft pairs crossed the dayside magnetopause at nearly the same universal time and magnetic local time but at much different latitudes. This fortuitous occurrence allows the magnetopause and flux transfer events (FTEs) to be studied at two widely separated sites simultaneously. FTEs are observed at both locations, those at UKS having standard normal component signatures, while those at ISEE have reverse signatures. The FTEs at UKS, closer to the equator, appear to have less helicity, or “twistedness” than those at ISEE far to the south. By identifying FTEs using the stringent Rijnbeek criterion for the bipolar Bn signature one might conclude that FTEs are not necessarily detected simultaneously at UKS and ISEE. However, careful analysis of the field strength behavior at ISEE does reveal evidence that FTEs are indeed observed at both sites simultaneously. While within the magnetosphere, AMPTE and ISEE both observe a coherent field rarefaction coupled with a tilt; we speculate that the signature is associated with time‐dependent dayside magnetopause reconnection.

Magnetotail flux ropes

ISEE‐3 magnetometer observations are presented that are consistant with flux ropes within the magnetotail. Three instances have been found. The flux ropes are identified by a strong magnetic field in their core, a bipolar magnetic field signature, and a double peaked field signature indicative of an off‐center crossing. The axes of the flux ropes lie nearly parallel to the magnetotail axis. Inside the flux ropes, a hot plasma streams tailward and there is an increase in the flux of energetic protons and the intensity of plasma waves. When the flux ropes were observed, ground magnetograms near local midnight were highly disturbed.