Showing papers by "Ramon Lopez published in 1993"

Pseudobreakup and substorm growth phase in the ionosphere and magnetosphere

Abstract: Observations made during the growth phase and the onset of a substorm on August 31, 1986 are presented. About 20 min after the epsilon parameter at the magnetopause had exceeded 10 exp 11 W, magnetic field dipolarization with an increase of energetic particle fluxes was observed by the AMPTE Charge Composition Explorer (CCE) spacecraft at the geocentric distance of 8.7 R(E) close to magnetic midnight. The event exhibited local signatures of a substorm onset at AMPTE CCE and a weak wedgelike current system in the midnight sector ionosphere, but did not lead to a full-scale substorm expansion; neither did it produce large particle injections at GEO. Only after another 20 min of continued growth phase could the entire magnetosphere-ionosphere system allow the onset of a regular substorm expansion. The initial activation is interpreted as a 'pseudobreakup'. We examine the physical conditions in the near-Earth plasma sheet and analyze the development in the ionosphere using ground-based magnetometers and electric field observations from the STARE radar.

A multisatellite study of a Pseudo-substorm onset in the near-earth magnetotail

Abstract: This paper reports the multisatellite and ground observations of two pseudo-substorm onset events that occurred successively at 0747 UT and 0811 UT, May 30, 1985, with more attention to the 0747 UT onset. The distinguishing features of the 0747 UT event are as follows. (1) The substorm-associated tail reconfiguration started in a very localized region in the near-Earth magnetotail. (2) The magnitude of the current disruption decreased markedly as the disruption region expanded tailward. (3) On the ground the onset of a very small negative bay (approx. 40 nT) was observed simultaneously with the onset of the current disruption, but over a much wider local time sector than the near-Earth tail reconfiguration. Positive bay onsets at mid-latitudes also had a longitudinally wide distribution. From these features we infer than in the present event the current disruption took place filamentarily near AMPTE/CCE at approx. 8.8 R(sub E). It is also inferred that pseudo-substorm onsets are distinguished from standard substorm onsets by the absence of a global expansion of the current disruption, and that the spatial scales of the onset region in the magnetosphere is not a major difference between the two. The present study suggests that the spatial distribution of the magnetic distortion before onsets is an important factor to determine the expansion scale of the current disruption. It is also suggested that the current disruption is basically an internal process of the magnetosphere.

CDAW 9 analysis of magnetospheric events on May 3, 1986: Event C

Abstract: An intense geomagnetic substorm event on May 3, 1986, occurring toward the end of a strong storm period, is studied. The auroral electrojet indices and global imaging data from both the Northern and Southern Hemispheres clearly revealed the growth phase and expansion phase development for a substorm with an onset at 0111 UT. An ideally located constellation of four spacecraft allowed detailed observation of the substorm growth phase in the near-tail region. A realistic time-evolving magnetic field model provided a global representation of the field configuration throughout the growth and early expansion phase of the substorm. Evidence of a narrowly localized substorm onset region in the near-earth tail is found. This region spread rapidly eastward and poleward after the 0111 UT onset. The results are consistent with a model of late growth phase formation of a magnetic neutral line. This reconnection region caused plasma sheet current diversion before the substorm onset and eventually led to cross-tail current disruption at the time of the substorm onset.

Simultaneous observation of the poleward expansion of substorm electrojet activity and the tailward expansion of current sheet disruption in the near-earth magnetotail

Abstract: A substorm that occurred on 7 June 1985 at 2209 UT for which simultaneous measurements from ground stations and CCE are available is considered. The event occurred during a close conjunction between CCE, the EISCAT magnetometer cross, and the STARE radar, allowing a detailed comparison of satellite and ground-based data. Two discrete activations took place during the first few minutes of this substorm: the expansion phase onset at 2209 UT and an intensification at 2212 UT, corresponding to a poleward expansion of activity. The energetic particle data indicate that the active region of the magnetotail during the 2212 UT intensification was located tailward of the active region at 2209 UT. This is direct evidence for a correspondence between tailward expansion of localized activity in the near-earth magnetotail (current disruption and particle energization) and poleward expansion of activity (electrojet formation) in the ionosphere.

Analysis of the c-FOS gene on chromosome 14 and the promoter of the amyloid precursor protein gene in familial alzheimer's disease

Abstract: The c-FOS gene product, a putative transacting transcriptional regulator of the amyloid precursor protein (APP) gene, is a candidate locus for the familial Alzheimer9s disease (FAD) mutation on chromosome 14 (FAD 14 ). In light of this functional relationship, we investigated the nucleotide sequence and segregation of c-FOS and the nucleotide sequence of the 59 APP promoter. Single-stranded conformational polymorphisms (SSCPs) in the c-FOS gene revealed that c-FOS closely cosegregates with the FAD 14 gene but does not show allelic association with FAD. A conservative third-position T→C mutation was demonstrated in exon 2 (codon 84) of c-FOS , and a C→G substitution was detected at—209 bp in the 59 promoter of APP. Neither were unique to FAD and are unlikely to be pathogenic or secondary modifiers of the FAD phenotype. We conclude that the c-FOS open reading frame is probably not the site of the FAD 14 locus, but we cannot exclude the existence of modifier loci on chromosome 21.

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

Abstract: 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.

Hybrid simulations of thin current sheets

Abstract: A one-dimensional, hybrid simulation code is used to study current sheets with a nonzero normal magnetic field B(sub z) and a dawn-to-dusk electric field E(sub y). Such configurations are dependent upon only two parameters: we use the normalized normal magnetic field B-normalized (sub z) = B(sub z)/(4(pi)(n(sub b)) (v(exp 2 sub T))(exp 1/2) and normalized electric field V-normalized (sub D) = (1/V(sub T)(cE(sub y)/B(sub z)), where V(sub T) is the thermal velocity of ions prior to their interaction with the current sheet and n(sub b) is the number density outside the current sheet (at the simulation boundary). A third parameter that is relevant to the motion of particles in current sheets is kappa(sub A), the value of kappa = (R(sub min)/rho(sub max))(exp 1/2) for particles of average energy. We find that if either B-normalized (sub z) is close to or greater than 1, or if kappa(sub A) is close to 1, a rotational mode develops in which the z = 0 current rotates with the ion sense about the normal magnetic field, while for small values of both B-normalized (sub z) or kappa(sub A), the configuration is quasi-steady. To achieve values of kappa(sub A) of the order of or larger than 1, we decrease the value of V-normalized (sub D) uniformly. We find that the magnetic field fluctuations and particle distribution functions are similar in many respects to what was observed in the day 240, 1986, Active Magnetospheric Particle Tracer Explorer (AMPTE)/CCE current disruption event, an event that appears to be located at the site of initiation of current disruption and related particle energization.

AGU participation in National Science and Technology Week

Abstract: During National Science and Technology Week (NSTW), which will be celebrated from April 25 through May 1, events are scheduled across the country to highlight science and technology. NSTW, sponsored by the National Science Foundation, presents a marvelous vehicle for increasing public understanding and interest in the branches of geoscience. The AGU Committee for Education and Human Resources requests AGU members to sponsor and carry out NSTW-related activities, such as giving a presentation to teachers describing their research. Interested individuals should obtain a file (lopez/nstw.txt) via anonymous FTP from avl.umd.edu, or copy.