L. F. Burlaga

Publications (103)97.2 Total impact

• Source

  Available from: I. G. Richardson

  Article: Simultaneous observations of solar MeV particles in a magnetic cloud and in the earth's northern tail lobe - Implications for the global field line topology of magnetic clouds and for the entry of solar particles into the magnetosphere during cloud passage

  C. J. Farrugia, I. G. Richardson, L. F. Burlaga, R. P. Lepping, V. A. Osherovich
  [show abstract] [hide abstract]
  ABSTRACT: Simultaneous ISEE 3 and IMP 8 spacecraft observations of magnetic fields and flow anisotropies of solar energetic protons and electrons during the passage of an interplanetary magnetic cloud show various particle signature differences at the two spacecraft. These differences are interpretable in terms of the magnetic line topology of the cloud, the connectivity of the cloud field lines to the solar surface, and the interconnection between the magnetic fields of the magnetic clouds and of the earth. These observations are consistent with a magnetic cloud model in which these mesoscale configurations are curved magnetic flux ropes attached at both ends to the sun's surface, extending out to 1 AU.
  10/1993;
• Article: Nonlinear evolution of magnetic flux ropes. I - Low-beta limit

  V. A. Osherovich, C. J. Farrugia, L. F. Burlaga
  [show abstract] [hide abstract]
  ABSTRACT: We study the nonlinear self-similar evolution of a cylindrical magnetic flux tube with two components of the magnetic field, axial and azimuthal. We restrict ourselves to the case of a plasma of low beta. Introducing a special class of configurations we call 'separable fields', we reduce the problem to an ordinary differential equation. Two cases are to be distinguished: (1) when the total field minimizes on the symmetry axis, the magnetic configuration inexorably collapses, and (2) when, on the other hand, the total field maximizes on the symmetry axis, the magnetic configuration behaves analogously to a nonlinear oscillator. Here we focus on the latter case. The effective potential of the motion contains two terms: a strong repulsive term and a weak restoring term associated with the pinch. We solve the nonlinear differential equation of motion numerically and find that the period of oscillations grows exponentially with the energy of the oscillator. Our treatment emphasizes the role of the force-free configuration as the lowest potential energy state about which the system oscillates.
  09/1993;
• Article: The earth's magnetosphere under continued forcing - Substorm activity during the passage of an interplanetary magnetic cloud

  C. J. Farrugia, M. P. Freeman, L. F. Burlaga, R. P. Lepping, K Takahashi
  [show abstract] [hide abstract]
  ABSTRACT: Magnetic field and energetic particle observations from six spacecraft in the near-earth magnetotail are described and combined with ground magnetograms to document for the first time the magnetospheric substorm activity during a 30-hour long transit of an interplanetary cloud at 1 AU. During an earlier 11-hr interval when B(z) was continuously positive, the magnetosphere was quiescent, while in a later 18-hr interval when B(z) was uninterruptedly negative a large magnetic storm was set off. In the latter interval the substorm onsets recurred on average every 50 min. Their average recurrence frequency remained relatively undiminished even when the magnetic cloud B(z) and other measures of the interplanetary energy input decreased considerably. These results concur with current models of magnetospheric substorms based on deterministic nonlinear dynamics. The substorm onset occurred when the cloud's magnetic field had a persistent northward component but was predominantly westward pointing.
  06/1993;
• Article: The interaction of a magnetic cloud with the Earth - Ionospheric convection in the Northern and Southern Hemispheres for a wide range of quasi-steady interplanetary magnetic field conditions

  M. P. Freeman, C. J. Farrugia, L. F. Burlaga, M. R. Hairston, M. E. Greenspan, J. M. Ruohoniemi, R. P. Lepping
  [show abstract] [hide abstract]
  ABSTRACT: Observations are presented of the ionospheric convection in cross sections of the polar cap and auroral zone as part of the study of the interaction of the Earth's magnetosphere with the magnetic cloud of January 13-15, 1988. For strongly northward IMF, the convection in the Southern Hemisphere is characterized by a two-cell convection pattern comfined to high latitudes with sunward flow over the pole. The strength of the flows is comparable to that later seen under southward IMF. Superimposed on this convection pattern there are clear dawn-dusk asymmetries associated with a one-cell convection component whose sense depends on the polarity of the magnetic cloud's large east-west magnetic field component. When the cloud's magnetic field turns southward, the convection is characterized by a two-cell pattern extending to lower latitude with antisunward flow over the pole. There is no evident interhemispheric difference in the structure and strength of the convection. Superimposed dawn-dusk asymmetries in the flow patterns are observed which are only in part attributable to the east-west component of the magnetic field.
  06/1993;
• Article: A study of an expanding interplanatary magnetic cloud and its interaction with the earth's magnetosphere - The interplanetary aspect

  C. J. Farrugia, L. F. Burlaga, V. A. Osherovich, I. G. Richardson, M. P. Freeman, R. P. Lepping, A. J. Lazarus
  [show abstract] [hide abstract]
  ABSTRACT: High time resolution interplanetary magnetic field and plasma measurements of an interplanetary magnetic cloud and its interaction with the earth's magnetosphere on January 14/15, 1988 are interpreted and discussed. It is argued that the data are consistent with the theoretical model of magnetic clouds as flux ropes of local straight cylindrical geometry. The data also suggest that this cloud is aligned with its axis in the ecliptic plane and pointing in the east-west direction. Evidence consisting of the intensity and directional distribution of energetic particle in the magnetic cloud argues in favor of the connectedness of the magnetic field lines to the sun's surface. The intensities of about 0.5 MeV ions is rapidly enhanced and the particles stream in a collimated beam along the magnetic field preferentially from the west of the sun. The particles travel form a flare site along the cloud magnetic field lines, which are thus presumably still attached to the sun.
  06/1993;
• Article: First results from the Giotto magnetometer experiment during the P/Grigg-Skjellerup encounter

  F. M. Neubauer, H. Marschall, M. Pohl, K.-H. Glassmeier, G. Musmann, F. Mariani, M. H. Acuna, L. F. Burlaga, N. F. Ness, M. K. Wallis
  [show abstract] [hide abstract]
  ABSTRACT: The Giotto magnetic field experiment has provided the first magnetic field data on the interaction between the solar wind and a low gas production comet, P/Grigg-Skjellerup. Waves produced by ion pick-up instabilities have been observed throughout the interaction region with particularly simple waveforms at large distances and a rich phenomenology. A bow shock has been observed outbound only, whereas inbound a change in the character of the wave fields occurred without a jump in the magnetic field vector. The inbound and outbound crossings of the bow wave and shock at 19,900 km and 25,400 km from the nucleus, respectively, imply a neutral gas production rate of (6.7 +/- 1.6) x 10 exp 27/sec. A magnetic field cavity of the comet was not crossed. The pile-up region of 2500 km width along the trajectory showed a magnetic field peak of 88.7 nT.
  03/1993;
• Article: Cosmic ray modulation and the distant heliospheric magnetic field - Voyager 1 and 2 observations from 1986 to 1989

  L. F. Burlaga, F. B. Mcdonald, N. F. Ness
  [show abstract] [hide abstract]
  ABSTRACT: The relation between the cosmic ray (CR) intensity and the magnetic field strength observed by Voyager 1 and 2 during the period from 1986 through most of 1989 beyond 19 AU is examined. It is shown that these observations generally confirm the relation between changes in the CR intensity and the magnetic field intensity observed beyond 10 AU from 1981 to 1985. The CR intensity decreases when merged interaction regions (MIRs) are strong and dominating, and it fluctuates about a plateau when the effects of corotating MIRs are balanced by rarefaction regions. The CR intensity increases when the MIRs are weak and the magnetic field strength is relatively low.
  02/1993;
• Article: Model of a rotating magnetic cloud

  C. J. Farrugia, V. A. Osherovich, L. F. Burlaga
  [show abstract] [hide abstract]
  ABSTRACT: The possibility that magnetic clouds rotate while they propagate antisunward was investigated. Magnetic clouds are modeled as magnetic flux ropes which rotate rigidly about the axis of symmetry. An ideal magnetohydrodynamic model, in which the evolution of the magnetic structure is related to the time evolution of the angular frequency, is developed. A class of 'separable' magnetic fields is employed to reduce the problem to a nonlinear ordinary differential equation for the evolution function, and it is solved numerically. The corresponding effective potential gives rise to two modes of evolution--expansion and oscillation--depending on the energy and on the value of a dimensionless parameter, k. Parameter k depends on the gas pressure, the ratio of the magnetic field components, and the frequency of rotation. There is a critical value of k, k(sub c), above which the oscillatory regime disappears and the flux rope invariably expands, regardless of the energy. Below k(sub c) the energy determines whether the configuration is confined or unbounded. Rotation always helps expansion by lowering the potential barrier. A data example was studied and features which are interpreted as signatures of rotation are presented. The angular speed is comparable to the Alfven speed, and the core of the rotating cloud completes on average one full revolution every three days at 1 AU. The parameter k is calculated from observations, and it is found to be close to, but below, critical. Only three out of the nine clouds examined showed signatures of rotation. Theoretical analysis suggests that close to the Sun rotation effects may play a more important role in the evolution of magnetic clouds than 1 AU.
  10/1992;
• Article: Neptune's polar cusp region - Observations and magnetic field analysis

  R. P. Lepping, L. F. Burlaga, A. J. Lazarus, V. M. Vasyliunas, A. Szabo, J. Steinberg, N. F. Ness, S. M. Krimigis
  [show abstract] [hide abstract]
  ABSTRACT: This paper confirms and extends the results of Szabo et al. (1991) (which demonstrated some similarities of the Neptune's polar cusp region to the earth's cusp), but uses a different approach requiring plasma and vector magnetic field quantities. In addition, various MHD properties of the cusp-magnetopause boundary, which separates the cusp from the magnetosheath allowing thermal anisotropy, are obtained, including the magnetopause (MP) normal, mass, and normal momentum flux, the boundary speed (and thickness), and their relationships. Results demonstrate that the MP velocity is composed of two components: a propagation speed and the other component consistent with the rotational motion of the magnetosphere.
  07/1992;
• Article: Magnetic cloud passage at Earth and associated substorm activity

  C. J. Farrugia, M. P. Freeman, L. F. Burlaga
  [show abstract] [hide abstract]
  ABSTRACT: An approach to the study of the solar wind-magnetosphere interaction by signal type, that is, by examining the effect in the magnetosphere of well defined interplanetary structures, is presented. Focus is on the response of the magnetosphere to interplanetary magnetic clouds. Among their properties are: the slow and smooth variation of the magnetic field vector, with fluctuation level well below common interplanetary values; the similarly well behaved bulk flow; the wide range of field and flow parameters; and the longevity of passage (1 to 2 days). If the magnetic cloud is oriented such that a long period of uninterruptedly northward pointing field is followed by a long interval of continuously southward pointing field, then the transition of the magnetosphere from a quiescent state (the 'ground state') to a very active state can be studied, the latter being sustained by continued forcing from the magnetic cloud. A synopsis of the main findings of a recent study in such an interaction is given, concentrating on the substorm activity attending the second part of cloud passage.
  07/1992;
• Article: Radial expansion of an ideal MHD configuration and the temporal development of the magnetic field

  C. J. Farrugia, V. A. Osherovich, L. F. Burlaga, R. P. Lepping, M. P. Freeman
  [show abstract] [hide abstract]
  ABSTRACT: We study the free radial expansion of a 3-component magnetic configuration. The emphasis of this paper is on the behavior of a field undergoing non-self-similar expansion. Comparing our results with the evolution of a magnetic configuration expanding self-similarly, we find that self-similar expansion appears as the asymptotic limit (with time) of the general case. Using a model field we show that a non-self-similar velocity profile need not have a strict monotonic decrease with time.
  02/1992;
• Article: A comparative study of dynamically expanding force-free, constant-alpha magnetic configurations with applications to magnetic clouds

  C. J. Farrugia, L. F. Burlaga, V. A. Osherovich, R. P. Lepping
  [show abstract] [hide abstract]
  ABSTRACT: We contrast two different solutions of the constant alpha, force-free MHD equation, both of which have been suggested as models for magnetic clouds: a solution in cylindrical coordinates and one in spherical coordinates. In line with the observation that magnetic clouds expand, we generalize these static models and construct their expanding counterparts. We find that expansion introduces in both cases a large asymmetry in the field strength signature which is in the same sense as that seen the the data, i.e. towards the leading edge of the cloud. We then do a least squares fit of the respective models to one-spacecraft data on a magnetic cloud. We find that the fitting routine converges in both cases. However, while purely formally we cannot distinguish between the two models using data from one spacecraft, the field components in the 'spherical' model have features not compatible with data on magnetic clouds.
  02/1992;
• Article: Structure and evolution of compound streams at not greater than 1 AU

  L. F. Burlaga, A. Hewish, K. W. Behannon
  [show abstract] [hide abstract]
  ABSTRACT: Two cases of compound stream formation and evolution at heliocentric distances of not greater than 1 AU have been studied using simultaneous data from both multipoint in situ and global sources. The sequences of events analyzed occurred around December 16, 1978, and February 19, 1979, respectively. The sources of the multipoint observations were Helios A, Helios B, IMP 8, and ISEE 3. The global observations were obtained by interplanetary scintillation analysis of radio sources. It was found that the radio and in situ data complemented each other and that both were needed for a unique interpretation of complex, time-dependent flows. The cases studied show that a single corotating stream can interact with different transient flows at different times, and a single transient flow can interact with different corotating streams at different times. In each instance, a compound stream is formed. This work further shows that the cosmic ray intensity at a point depends on the global compound stream configuration in the vicinity of the point.
  01/1992;
• Article: Giotto's mission to planet earth

  K.-H. Glassmeier, F. M. Neubauer, G. Brach, H. Marshall, M. H. Acuna, L. F. Burlaga, F. Mariani, G. Musmann, N. F. Ness, M. K. Wallis
  [show abstract] [hide abstract]
  ABSTRACT: The Giotto spacecraft was reactivated in February 1990 and performed the first-ever earth gravity-assisted maneuver on July 2, 1990 to be retargeted for Comet P/Grigg-Skjellerup. This swing-by is of unique scientific interest due to Giotto's hyperbolic, high-inclination orbit. This paper reports on scientific results of the Giotto magnetic field experiment. Due to the high fly-by velocity and the relative quietness of the magnetosphere during the swing-by period, these measurements present a snapshot view of the earth magnetosphere with clearly identified inbound and outbound bow shock and magnetopause crossings. The outbound crossings are of particular interest as surface waves at the polar magnetopause at a distance of 28 earth radii as well as a strong quasi-perpendicular bow shock at a distance of about 64 earth radii are observed.
  10/1991;
• Article: The interaction of a very large interplanetary magnetic cloud with the magnetosphere and with cosmic rays

  R. P. Lepping, L. F. Burlaga, K. W. Ogilvie, B. T. Tsurutani, A. J. Lazarus
  [show abstract] [hide abstract]
  ABSTRACT: The observation of one of the largest magnetic clouds ever observed at a distance of 1 AU, with a diameter of greater than about 0.4 AU, is reported. The cloud is shown to be almost unchanged structurally by interaction with the earth bow shock. The first observations are reported of an auroral activity response to the passage of a magnetic cloud, with a nearly immediate increase in auroral activity when the IMF theta(B) angle reversed polarity to negative near the cloud center. The results provide strong evidence that turbulent magnetic fields behind interplanetary shocks are a possible cause of Forbush decreases, but contest the idea that relatively smooth, strong fields in clouds are a cause of such decreases. The cloud field modeling supports the existence of magnetic force-free fields in describing cloud structure.
  07/1991;
• Article: Cosmic ray modulation - Voyager 2 observations, 1987-1988

  L. F. Burlaga, F. B. Mcdonald, N. F. Ness, A. J. Lazarus
  [show abstract] [hide abstract]
  ABSTRACT: The intensity profile of cosmic rays above 70 MeV observed by Voyager 2 and its relation to the interplanetary magnetic field and plasma at the beginning of the new modulation cycle from day 190, 1987 to day 345, 1988 in the region from 23.3 AU to 27.8 AU is analyzed. The cosmic ray intensity profile was approximately a series of four plateaus separated by three steps in which the intensity dropped abruptly. Each step was associated with a region in which the magnetic field, density and temperature were higher than average. The plateaus were associated with regions in which the magnetic field was alternately strong and weak. The solar wind within 200 AU during this interval can be roughly pictured as consisting of three shells between which the flow was quasiperiodic with a 26 day periodicity. The latitudinal extent of the shells in the northern hemisphere was probably less than 33 deg, since no steps were observed by Voyager 1. Drift motions might play a role during the recovery phase, just prior to the onset of the new modulation cycle, in the plateau regions between the shells, within the shells where drifts in various directions might mimic diffusion, and close to 1 AU, where large regions of intense magnetic fields have not yet formed. However the principal decreases in the cosmic ray intensity in the outer heliosphere during 1987 and 1988 were associated with the passage of broad regions of intense magnetic fields, consistent with the diffusion/convection model.
  04/1991;
• Article: Energetic ion observations in the magnetic cloud of 14-15 January 1988 and their implications for the magnetic field topology

  I. G. Richardson, C. J. Farrugia, L. F. Burlaga
  [show abstract] [hide abstract]
  ABSTRACT: On 14-15 January 1988, a magnetic cloud with a local field topology consistent with an east-west aligned cylindrical flux-rope and which formed the driver of an interplanetary shock passed the earth. Using 0.5-4 MeV/n ion data from the instrument on IMP 8, the paper addresses the question of whether or not magnetic field lines within the magnetic cloud were connected to the sun. An impulsive solar particle event was detected inside the magnetic cloud strongly suggesting that the field lines were rooted at the sun.
  02/1991;
• Article: Pressure-balanced structures between 1 AU and 24 AU and their implications for solar wind electrons and interstellar pickup ions

  L. F. Burlaga, J. D. Scudder, L. W. Klein, P. A. Isenberg
  [show abstract] [hide abstract]
  ABSTRACT: 'Pressure-balanced structures' (PBSs) in the heliosphere are microscale phenomena with a characteristic length along the radial dimension of the order of 0.05 AU across which the sum of the thermal pressures and the magnetic pressure is constant. PBSs have been identified in the Voyager data from 1 AU to 24 AU. If the density of pickup ions were proportional to the proton density across a PBS, and if the electron pressure were negligible, then from measurements of the magnetic field and the proton density and temperature across pressure balanced structures a pickup ion density is derived which is significantly smaller than the density predicted by a model based on the same assumption. This suggests that there are processes which 'smear out' the pressure of the pickup ions so that it is nearly constant on the scale of a PBS. If the pressure of the pickup ions is uniform across a PBS, then one can determine the electron temperature from the pressure balance condition.
  04/1990;
• Article: Spatial variation and evolution of heliospheric sector structure

  K. W. Behannon, L. F. Burlaga, J. T. Hoeksema, L. W. Klein
  [show abstract] [hide abstract]
  ABSTRACT: The magnetic sector polarity at the sun and in the IMF was surveyed during about three quarters of sunspot cycle 21 using ground-based photospheric magnetic field observations and spacecraft observations, including Voyagers 1 and 2 and Pioneer Venus Orbiter data. The location of the heliospheric current sheet near the sun throughout the period 1977-1985 is calculated. The large-scale magnetic polarity structure was in almost continuous evolution throught this period. IMF polarity patterns derived from spacecraft data are compared with the pattern observed at the sun, showing that sector pattern stability decreases with increasing heliocentric distance.
  03/1989;
• Article: Meridional plasma flow in the outer heliosphere

  A. J. Lazarus, B. Yedidia, L. Villanueva, Mcnutt, R. L, J. W. Belcher, U. Villante, L. F. Burlaga
  [show abstract] [hide abstract]
  ABSTRACT: Voyager 2 observations made in the outer heliosphere near 25 AU and within 2 deg of the heliographic equatorial plane show periodic variations in the meridional (North/South) flow velocities that are much more prominent than the East/West variations. An autocorrelation analysis shows that the flow variation has a period of about 25.5 days in the latter half of 1986, in approximate agreement with the solar rotation period. The results suggest that increased pressure in interaction regions remains the best candidate for the driver of the nonradial flows.
  01/1989;