scholarly journals Solitary waves observed in the auroral zone: the Cluster multi-spacecraft perspective

2004 ◽  
Vol 11 (2) ◽  
pp. 183-196 ◽  
Author(s):  
J. S. Pickett ◽  
S. W. Kahler ◽  
L.-J. Chen ◽  
R. L. Huff ◽  
O. Santolík ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Solitary Waves ◽  
Correlation Study ◽  
Auroral Zone ◽  
Potential Change ◽  
Local Magnetic Field ◽  
Double Layers ◽  
Magnetic Field Direction ◽  
Field Lines ◽  
Generation Mechanisms

Abstract. We report on recent measurements of solitary waves made by the Wideband Plasma Wave Receiver located on each of the four Cluster spacecraft at 4.5-6.5RE (well above the auroral acceleration region) as they cross field lines that map to the auroral zones. These solitary waves are observed in the Wideband data as isolated bipolar and tripolar waveforms. Examples of the two types of pulses are provided. The time durations of the majority of both types of solitary waves observed in this region range from about 0.3 up to 5ms. Their peak-to-peak amplitudes range from about 0.05 up to 20mV/m, with a few reaching up to almost 70mV/m. There is essentially no potential change across the bipolar pulses. There appears to be a small, measurable potential change, up to 0.5V, across the tripolar pulses, which is consistent with weak or hybrid double layers. A limited cross-spacecraft correlation study was carried out in order to identify the same solitary wave on more than one spacecraft. We found no convincing correlations of the bipolar solitary waves. In the two cases of possible correlation of the tripolar pulses, we found that the solitary waves are propagating at several hundred to a few thousand km/s and that they are possibly evolving (growing, decaying) as they propagate from one spacecraft to the next. Further, they have a perpendicular (to the magnetic field) width of 50km or greater and a parallel width of about 2-5km. We conclude, in general, however, that the Cluster spacecraft at separations along and perpendicular to the local magnetic field direction of tens of km and greater are too large to obtain positive correlations in this region. Looking at the macroscale of the auroral zone at 4.5-6.5RE, we find that the onsets of the broadband electrostatic noise associated with the solitary waves observed in the spectrograms of the WBD data are generally consistent with propagation of the solitary waves up the field lines (away from Earth), or with particles or waves propagating up the field line, which leads to local generation of the solitary waves all along the field lines. A discussion of the importance of these solitary waves in magnetospheric processes and their possible generation mechanisms, through electron beam instabilities and turbulence, is provided.

scholarly journals On the generation of solitary waves observed by Cluster in the near-Earth magnetosheath

2005 ◽  
Vol 12 (2) ◽  
pp. 181-193 ◽  
Author(s):  
J. S. Pickett ◽  
L.-J. Chen ◽  
S. W. Kahler ◽  
O. Santolík ◽  
M. L. Goldstein ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Solitary Waves ◽  
Bow Shock ◽  
Generation Mechanism ◽  
Local Magnetic Field ◽  
Magnetic Field Direction ◽  
Waveform Data ◽  
Background Magnetic Field ◽  
Ion Velocity ◽  
The Magnetic Field

Abstract. Through case studies involving Cluster waveform observations, solitary waves in the form of bipolar and tripolar pulses have recently been found to be quite abundant in the near-Earth dayside magnetosheath. We expand on the results of those previous studies by examining the distribution of solitary waves from the bow shock to the magnetopause using Cluster waveform data. Cluster's orbit allows for the measurement of solitary waves in the magnetosheath from about 10 RE to 19.5 RE. Our results clearly show that within the magnetosheath, solitary waves are likely to be observed at any distance from the bow shock and that this distance has no dependence on the time durations and amplitudes of the solitary waves. In addition we have found that these same two quantities show no dependence on either the ion velocity or the angle between the ion velocity and the local magnetic field direction. These results point to the conclusion that the solitary waves are probably created locally in the magnetosheath at multiple locations, and that the generation mechanism is most likely not solely related to ion dynamics, if at all. To gain insight into a possible local generation mechanism, we have examined the electron differential energy flux characteristics parallel and perpendicular to the magnetic field, as well as the local electron plasma and cyclotron frequencies and the type of bow shock that Cluster is behind, for several time intervals where solitary waves were observed in the magnetosheath. We have found that solitary waves are most likely to be observed when there are counterstreaming (~parallel and anti-parallel to the magnetic field) electrons at or below about 100eV. However, there are times when these counterstreaming electrons are present when solitary waves are not. During these times the background magnetic field strength is usually very low (<10nT), implying that the amplitudes of the solitary waves, if present, would be near or below those of other waves and electrostatic fluctuations in this region making it impossible to isolate or clearly distinguish them from these other emissions in the waveform data. Based on these results, we have concluded that some of the near-Earth magnetosheath solitary waves, perhaps in the form of electron phase-space holes, may be generated locally by a two-stream instability involving electrons based on the counterstreaming electrons that are often observed when solitary waves are present. We have not ruled out the possibility that the solitary waves could be generated as a result of the lower-hybrid Buneman instability in the presence of an electron beam, through the electron acoustic mode or through processes involving turbulence, which is almost always present in the magnetosheath, but these will be examined in a more comprehensive study in the future.

Observations of auroral zone processes by the Viking satellite

1989 ◽  
Vol 328 (1598) ◽  
pp. 209-220 ◽  
Keyword(s):  
Magnetic Field ◽  
Auroral Oval ◽  
Auroral Zone ◽  
Double Layers ◽  
Particle Interactions ◽  
Imaging Experiment ◽  
Auroral Electrons ◽  
Field Lines ◽  
Scientific Results ◽  
The Magnetic Field

The scientific results of the Viking project obtained up to the spring of 1988 are reviewed. During solar minimum conditions, when Viking was operated, the dayside auroral oval has been found to be the most active part, except during strong substorms and storms. A number of new auroral morphological features have been seen with the imaging experiment onboard Viking. Large-amplitude slow fluctuations of the electric field heat the ionospheric plasma and pump up the magnetic moment of the ionospheric ions so that they may leave the ionosphere. These fluctuations also accelerate ionospheric electrons upwards along the magnetic field lines. The importance of the acceleration of auroral electrons into the atmosphere by magnetic field-aligned potential differences has been confirmed. The first satellite-borne plasma wave interferometer on Viking has made it possible to determine a number of characteristics of the ‘weak’ double layers, seen first by the S3-3 satellite. A large number of these along the magnetic field lines produce large electric potential differences. Many new results concerning wave-particle interactions have been obtained, of which a few are presented here.

scholarly journals Modeling stretched solitary waves along magnetic field lines

2002 ◽  
Vol 9 (2) ◽  
pp. 101-109 ◽  
Author(s):  
L. Muschietti ◽  
I. Roth ◽  
C. W. Carlson ◽  
M. Berthomier
Keyword(s):  
Magnetic Field ◽  
Solitary Waves ◽  
Three Dimensional ◽  
Cylindrical Symmetry ◽  
Auroral Zone ◽  
Distribution Functions ◽  
Bgk Model ◽  
Gaussian Shape ◽  
Drift Frequency ◽  
Field Lines

Abstract. A model is presented for a new type of fast solitary waves which is observed in downward current regions of the auroral zone. The three-dimensional, coherent structures are electrostatic, have a positive potential, and move along the magnetic field lines with speeds on the order of the electron drift. Their parallel potential profile is flattened and cannot fit to the Gaussian shape used in previous work. We develop a detailed BGK model which includes a flattened potential and an assumed cylindrical symmetry around a centric magnetic field line. The model envisions concentric shells of trapped electrons slowly drifting azimuthally while bouncing back and forth in the parallel direction. The electron dynamics is analysed in terms of three basic motions that occur on different time scales characterized by the cyclotron frequency We , the bounce frequency wb , and the azimuthal drift frequency wg. The ordering We >> wb >> wg is required. Self-consistent distribution functions are calculated in terms of approximate constants of motion. Constraints on the parameters characterizing the amplitude and shape of the stretched solitary wave are discussed.

scholarly journals Magnetic field configuration and field-aligned acceleration of energetic ions during substorm onsets

2001 ◽  
Vol 19 (9) ◽  
pp. 1089-1094 ◽  
Author(s):  
A. Korth ◽  
Z. Y. Pu
Keyword(s):  
Magnetic Field ◽  
Electric Fields ◽  
Plasma Sheet ◽  
Field Configuration ◽  
Local Magnetic Field ◽  
Magnetospheric Configuration And Dynamics ◽  
Night Side ◽  
Field Lines ◽  
Synchronous Orbit ◽  
Substorm Onsets

Abstract. In this paper, we present an interpretation of the observed field-aligned acceleration events measured by GEOS-2 near the night-side synchronous orbit at substorm onsets (Chen et al., 2000). We show that field-aligned acceleration of ions (with pitch angle asymmetry) is closely related to strong short-lived electric fields in the Ey direction. The acceleration is associated with either rapid dipolarization or further stretching of local magnetic field lines. Theoretical analysis suggests that a centrifugal mechanism is a likely candidate for the parallel energization. Equatorward or anti-equatorward energization occurs when the tail current sheet is thinner tailward or earthward of the spacecraft, respectively. The magnetic field topology leading to anti-equatorward energization corresponds to a situation where the near-Earth tail undergoes further compression and the inner edge of the plasma sheet extends inwards as close as the night-side geosynchronous altitudes.Key words. Magnetospheric physics (magnetospheric configuration and dynamics; plasma sheet; storms and sub-storms)

Solitary waves and double layers on auroral field lines

1983 ◽  
Vol 88 (A2) ◽  
pp. 916 ◽  
Author(s):  
M. K. Hudson ◽  
W. Lotko ◽  
I. Roth ◽  
E. Witt
Keyword(s):  
Solitary Waves ◽  
Double Layers ◽  
Field Lines

scholarly journals Electron acoustic solitary waves with non-thermal distribution of electrons

2004 ◽  
Vol 11 (2) ◽  
pp. 275-279 ◽  
Author(s):  
S. V. Singh ◽  
G. S. Lakhina
Keyword(s):  
Solitary Waves ◽  
Auroral Zone ◽  
Thermal Electron ◽  
Electrons And Ions ◽  
Cold Electrons ◽  
Field Lines ◽  
Unmagnetized Plasma ◽  
Zone Field ◽  
Electron Acoustic ◽  
Pseudo Potential

Abstract. Electron-acoustic solitary waves are studied in an unmagnetized plasma consisting of non-thermally distributed electrons, fluid cold electrons and ions. The Sagdeev pseudo-potential technique is used to carry out the analysis. The presence of non-thermal electrons modifies the parametric region where electron acoustic solitons can exist. For parameters representative of auroral zone field lines, the electron acoustic solitons do not exist when either α > 0.225 or Tc/Th > 0.142, where α is the fractional non-thermal electron density, and Tc (Th) represents the temperature of cold (hot) electrons. Further, for these parameters, the simple model predicts negatively charged potential structures. Inclusion of an electron beam in the model may provide the positive potential solitary structures.

scholarly journals Measuring Magnetic Fields from Water Masers Associated with a Synchrotron Protostellar Jet

2017 ◽  
Vol 13 (S336) ◽  
pp. 215-218
Author(s):  
Ciriaco Goddi ◽  
Gabriele Surcis
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Zeeman Splitting ◽  
High Mass ◽  
Local Magnetic Field ◽  
Mass Star ◽  
Dominant Component ◽  
Field Lines ◽  
Water Masers ◽  
The Magnetic Field

AbstractThe Turner-Welch Object in the W3(OH) high-mass star forming complex drives a synchrotron jet, which is quite exceptional for a high-mass protostar, and is associated with a strongly polarized water maser source, W3(H2O), making it an optimal target to investigate the role of magnetic fields on the innermost scales of protostellar disk-jet systems. We report here full polarimetric VLBA observations of water masers. The linearly polarized emission from water masers provides clues on the orientation of the local magnetic field, while the measurement of the Zeeman splitting from circular polarization provides its strength. By combining the information on the measured orientation and strength of the magnetic field with the knowledge of the maser velocities, we infer that the magnetic field evolves from having a dominant component parallel to the outflow velocity in the pre-shock gas (with field strengths of the order of a few tens of mG), to being mainly dominated by the perpendicular component (of order of a few hundred of mG) in the post-shock gas where the water masers are excited. The general implication is that in the undisturbed (i.e. not-shocked) circumstellar gas, the flow velocities would follow closely the magnetic field lines, while in the shocked gas the magnetic field would be re-configured to be parallel to the shock front as a consequence of gas compression.

scholarly journals Spiral structures and regularities in magnetic field variations and auroras

2012 ◽  
Vol 3 (1) ◽  
pp. 1-31 ◽  
Author(s):  
Y. I. Feldstein ◽  
L. I. Gromova ◽  
M. Förster ◽  
A. E. Levitin
Keyword(s):  
Magnetic Field ◽  
Geomagnetic Field ◽  
Current System ◽  
Temporal Distribution ◽  
Auroral Oval ◽  
Auroral Zone ◽  
Upper Atmosphere ◽  
Polar Coordinates ◽  
Magnetospheric Substorms ◽  
Field Lines

Abstract. The conception of spiral shaped precipitation regions, where solar corpuscles penetrate the upper atmosphere, was introduced into geophysics by C. Störmer and K. Birkeland at the beginning of the last century. Later, in the course of the XX-th century, spiral distributions were disclosed and studied in various geophysical phenomena. Most attention was devoted to spiral shapes in the analysis of regularities pertaining to the geomagnetic activity and auroras. We review the historical succession of perceptions about the number and positions of spiral shapes, that characterize the spatial-temporal distribution of magnetic disturbances. We describe the processes in the upper atmosphere, which are responsible for the appearance of spiral patterns. We considered the zones of maximal aurora frequency and of maximal particle precipitation intensity, as offered in the literature, in their connection with the spirals. We discuss the current system model, that is closely related to the spirals and that appears to be the source for geomagnetic field variations during magnetospheric substorms and storms. The currents in ionosphere and magnetosphere constitute together with field-aligned (along the geomagnetic field lines) currents (FACs) a common 3-D current system. At ionospheric heights, the westward and eastward electrojets represent characteristic elements of the current system. The westward electrojet covers the longitudinal range from the morning to the evening hours, while the eastward electrojet ranges from afternoon to near-midnight hours. The polar electrojet is positioned in the dayside sector at cusp latitudes. All these electrojets map along the magnetic field lines to certain plasma structures in the near-Earth space. The first spiral distribution of auroras was found based on observations in Antarctica for the nighttime-evening sector (N-spiral), and later in the nighttime-evening (N-spiral) and morning (M-spiral) sectors both in the Northern and Southern Hemispheres. The N- and M-spirals drawn in polar coordinates form an oval, along which one observes most often auroras in the zenith together with a westward electrojet. The nature of spiral distributions in geomagnetic field variations was unabmibuously interpreted after the discovery of the spiral's existence in the auroras had been established and this caused a change from the paradigm of the auroral zone to the paradigm of the auroral oval. Zenith forms of auroras are found within the boundaries of the auroral oval. The oval is therefore the region of most frequent precipitations of corpuscular fluxes with auroral energy, where anomalous geophysical phenomena occur most often and with maximum intensity. S. Chapman and L. Harang identified the existence of a discontinuity at auroral zone latitudes (Φ ∼ 67°) around midnight between the westward and eastward electrojets, that is now known as the Harang discontinuity. After the discovery of the auroral oval and the position of the westward electrojet along the oval, it turned out, that there is no discontinuity at a fixed latitude between the opposite electrojets, but rather a gap, the latitude of which varies smoothly between Φ ∼ 67° at midnight and Φ ∼ 73° at 20:00 MLT. In this respect the term ''Harang discontinuity'' represents no intrinsic phenomenon, because the westward electrojet does not experience any disruption in the midnight sector but continues without breaks from dawn to dusk hours.

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