Boundary Of Cusp Research Articles

Collisionless Plasma Processes at Magnetospheric Boundaries: Role of Strong Nonlinear Wave Interactions

The sunward Poynting flux throughout the magnetosheath and foreshock (directly measured by INERBALL-1, CLUSTER-4, and DOUBLE STAR TC1) and its correlation and bi-correlation with the dynamic pressure of the solar plasma flow have been analyzed. It has been demonstrated for the first time that perturbations caused by resonances in the magnetospheric boundary layers propagate upstream towards the bow shock as the short impulses of the sunward Poynting flux, which excite the strongest three-wave resonances. They are initiated in the foreshock and regulate the bow shock surface oscillations. Another interaction zone near the magnetopause assists plasma flow extra deflection and acceleration around the magnetopause. At the outer boundary of stagnant cusp the turbulent barrier can separate the flowing and stagnant plasmas namely by the three-wave cascades. Therefore, both experiment and magnetohydrodynamic simulation demonstrate the leading role of the discovered waves and nonlinear processes in the collisionless interaction of the plasma flow and magnetic barrier.

Field-Aligned Electrons in Polar Region Observed by Cluster on 30 September 2001

The physical process in the Earth's polar region is very complex and still needs to be further studied. Using the data from Cluster satellite measurement, an analysis on field-aligned electrons in the mid-latitude cusp on 30 September 2001 has been performed. The satellite observed a down-flowing electron event in the low-latitude boundary and a sequential up-flowing electron event in the high-latitude boundary of the cusp region. The down-flowing electron had a velocity as high as 500 km/s and a flux of 2.0 × 109 cm−2·s−1. The up-flowing electron had a velocity up to 1200 km/s and a flux of 4.9 × 109 cm−2·s−1. Both the velocity and the flux observed in this event are the maximum values of the up-flowing electrons observed by all satellites to date. The electron is the main contributor for the field-aligned current in this event. The physical mechanism is also discussed. The down-flowing electron in the low-latitude boundary of the cusp region may result from solar wind injecting during the southward IMF, and the up-flowing electrons in the high-latitude boundary of cusp may result from mirroring of the solar wind, or from the ionospheric up-flowing electrons which have been accelerated.

On the traces of Sobolev functions on the boundary of an anisotropic cusp

Given a “zero” cusp \(G_{\vec \lambda } \) with an anisotropic Holder singularity at the vertex, we consider various embedding theorems for the Sobolev spaces \(W_p^1 (G_{\vec \lambda } )\) and the questions of embeddings of the traces of Sobolev functions into Lebesgue classes on the boundary of the cusp.

Hyperbolic cusps with convex polyhedral boundary

We prove that a 3-dimensional hyperbolic cusp with convex polyhedral boundary is uniquely determined by the metric induced on its boundary. Furthemore, any hyperbolic metric on the torus with cone singularities of positive curvature can be realized as the induced metric on the boundary of a convex polyhedral cusp. The proof uses the total scalar curvature functional on the space of ``cusps with particles'', which are hyperbolic cone-manifolds with the singular locus a union of half-lines. We prove, in addition, that convex polyhedral cusps with particles are rigid with respect to the induced metric on the boundary and the curvatures of the singular locus. Our main theorem is equivalent to a part of a general statement about isometric immersions of compact surfaces.

Are small-scale field-aligned currents and magnetosheath-like particle precipitation signatures of the same low-altitude cusp?

We examined some 75 observations from the low-altitude Earth orbiting DMSP, Ørsted and CHAMP satellites which were taken in the region of the nominal cusp. Our objective was to determine whether the actually observed cusp locations as inferred from magnetosheath-like particle precipitation (“particle cusp”) and intense small-scale magnetic field variations (“current cusp”), respectively, were identical and were consistent with the statistically expected latitude of the cusp derived from a huge number of charged particle spectrograms (“statistical cusp”). The geocentric coordinates of the satellites were converted into AACGM coordinates, and the geomagnetic latitude of the cusp boundaries (as indicated by precipitating particles and small-scale field-aligned currents) set in relation to the IMF-B z dependent latitude of the equatorward boundary of the statistical cusp. We find that the actually observed latitude of the particle cusp matches well the statistically expected latitude while the current cusp appears to cover most of the statistical cusp and also a ≈1° wide section beyond the equatorward boundary of the statistical cusp. This leads us to suggest that intense small-scale field-aligned currents are generated in the cusp but also in the transition zone between the low-latitude boundary layer (LLBL) and the cusp, probably within both regimes, the cusp and the open LLBL. The small-scale field-aligned currents are possibly a consequence of turbulence and/or instabilities associated with the process of opening previously closed magnetospheric field lines and merging them with the interplanetary magnetic field.

On the traces of Sobolev functions on the boundary of a cusp with a Hölder singularity

For the Sobolev classes W p 1 on a “zero” cusp with a Holder singularity at the vertex, we consider the question of compactness of the embedding of the traces of Sobolev functions into the Lebesgue classes on the boundary of the cusp.

High-latitude ionosphere structure in dayside sector on ground-based and satellite measurements

We report electron density measurements from satellite Cosmos-900 the presence of high-latitude dayside troughs. The trough minima near noon are localized at 70–74° invariant latitute during indisturbed periods and at 60–70° invariant latitude during disturbed periods. The localization and dynamics of equatorial boundary of dayside polar cusp by oblique sounding of F-region from ground-based stations and by simultaneous measurements of precipitating particles from satellite are revealed. A comparison of statistical data of dayside troughs and reflections from dayside cusp shows that a trough minimum near noon is localized by 3–4° equatorward of the cusp. The obtained results are discussed.

Multiple‐Branch Model of the open magnetopause

When the magnetosphere is magnetically open, the magnetopause must cut across open field lines and is usually identified with a rotational discontinuity or finite‐amplitude Alfvén wave. The merging of magnetospheric and interplanetary magnetic fields in the nose region of the magnetosphere generates two pairs of such Alfvén waves; the two interior ones propagate down to the ionosphere, and the two exterior ones propagate into the incoming magnetosheath plasma and thus can be identified with the magnetopause in the dayside. However, continuing this magnetopause into the nightside magnetotail region would require reversing the direction of magnetic‐field rotation at some point on the propagating wave, which is not possible. To resolve this problem, I propose that the magnetopause on the nightside is not a continuation of that on the dayside but is a distinct surface formed by a separate Alfvén wave generated near the downstream edge of the dayside polar cusp. This multiple‐branch model also explains the boundary of the exterior cusp (identified with the continuation of the dayside magnetopause) and accounts for the non‐stagnant flow within the exterior cusp (the flow simply proceeds across the nightside magnetopause into the plasma mantle region). Where the two branches are adjacent to the magnetotail, their spatial separation can be estimated as typically no more than about 2 RE, not likely to have been noticed unless specifically looked for. A possible mechanism for generating the nightside Alfvén wave is the expansion of plasma within the polar cusp as the flux tubes are advected into the rapidly increasing volume of the magnetotail.

Impulsive electrostatic waves and field‐aligned currents observed in the entry layer

Impulsive electrostatic waves have been observed in regions of downward‐flowing region 1 Birkeland currents in data acquired by the High Frequency Wave and Magnetic Field instruments on the Viking satellite. These data were obtained at approximately 11,000 km altitude and in the 1100 MLT and 76° invariant latitude sector near the equatorward boundary of the dayside polar cusp. Wave and magnetic field data are presented from Viking orbit 1219 on October 1, 1986, and wave, magnetic field, and particle data acquired with the Hot Plasma instrument from orbit 1197 on September 27, 1986. The high frequency waves are observed between the electron plasma and cyclotron frequencies. Since the broadband electrostatic waves extend in frequency above the cold plasma frequency, a nonlinear treatment must be made to explain the observation. We suggest that electron acoustic waves may couple with other electron plasma waves, resulting in the distribution of wave power over a wide range of frequencies. A strong correlation exists between large‐scale downward‐flowing Birkeland currents (region 1 system) and impulsive electrostatic waves. The Viking Magnetic Field experiment allows one to resolve, in high time‐resolution, electron beams that may be unstable to the impulsive electrostatic waves.

High-latitude Pc1–2 geomagnetic pulsations and their connection with location of the dayside polar cusp

The result of investigating high-latitude Pc1–2 pulsations are presented in this paper. They show that these unstructured oscillations are typical in intervals of low magnetic activity for regions of projections of the dayside cusp on the Earth's surface. The morphological properties of these pulsations, namely the character of their diurnal variations and dependence of their amplitude and frequency of occurrence on magnetic activity on different latitudes, suggest methods of utilization for tracing the location of the equatorial boundary of the dayside cusp. It is suggested that Pc1–2 pulsations are generated mainly in the dayside magnetosheath on field lines, crossing the magnetopause and entering in the dayside cusp. The possible mechanism of generation is the ion-cyclotron instability of plasma of finite pressure ( β ⊥ ≳ 1) and with anisotropic temperature ( T ⊥ > T ∥).

Sudden commencement absorption events at the edge of the polar cap

Examination of riometer records from a 5-year period (1967–1971) at Godhavn, Greenland (77.2° invariant latitude), shows the presence of sudden commencement absorption on two occasions, both during the late spring and early summer months in the northern hemisphere. One event occurred during quiet geomagnetic conditions when the interplanetary field was northward; the other event occurred during disturbed conditions when the predicted position of the equatorward boundary of the polar cusp was 2° equatorward of the observing site.

Possibility of Drift-cyclotron Instabilities on the Boundary of the Polar Cusp

THE interaction between the solar wind plasma and the geomagnetic field produces two neutral regions on the noon-midnight meridian, one in the Northern Hemisphere and one in the Southern Hemisphere. The neutral region has been termed as polar cusp or cleft. From poleward of the cusp the geomagnetic field lines are swept back into the geomagnetic tail. The possible direct penetration of solar wind plasma through the polar cusp into the polar ionosphere has been inferred1,2. The boundary of the polar cusp is a particularly interesting region because of the great variety of plasma instabilities that are excited there and because of the roles that these play in auroral phenomena. The possibility of drift-cyclotron instabilities on the boundary of the polar cusp is discussed here.

Observed relationships between electric fields and auroral particle precipitation

Simultaneous electric field and plasma observations with the low-altitude polar-orbiting satellite Injun 5 have provided a comprehensive survey of convection electric fields and their association with magnetospheric plasma phenomena. The most prominent features of the convection electric fields are reversals located at high magnetic latitudes, with generally antisunward convection poleward and sunward convection equatorward of the electric field reversal location. The electric field reversal is interpreted as the boundary between open and closed magnetic field lines. During local day the electric field reversal is observed to coincide with the equatorward boundary of the polar cusp. The plasma flow in the dayside polar cusp region is dominantly E-W, away from the stagnation point, the convection velocities typically being about 1 km/sec. At local evening, 'inverted V' electron precipitation bands are observed near or at the position of the electric field reversal. In the local late-evening sector the electric field reversal becomes less distinct, and often no single well-defined electric field reversal can be identified. In all cases the inverted V electron precipitation events are closely associated with large, typically greater than 30 mV/m, irregular electric field fluctuations with time scales of a few seconds or less.

Equatorward shift of the polarFlayer irregularity zone as a function of theKpindex

Summary. Vertical incidence ionograms, recorded in 1968, 1969, and 1970 on the Air Force Cambridge Research Laboratories (AFCRL) flying ionospheric laboratory, have been used to study the polar F layer irregularity zone (Fliz) in the geomagnetic day sector. The Fliz has been shown to be most probably caused by precipitation of particles through the polar cusp. A latitudinal sequence of ionograms is presented to demonstrate the typical abrupt transition, which occurs in less than 1° of latitude, from a ‘normal’ midlatitude F layer reflection (that is, a reflection that shows both O and X components and stratification into F1 and F2 layer) to the very irregular and diffuse reflections characteristic of the Fliz. This abrupt transition, which marks the equatorward boundary of the Fliz, occurs on all 30 flights through the geomagnetic day sector. A dependence of the equatorward boundary of the Fliz on corrected geomagnetic (CG) local time and on Kp is shown. In the geomagnetic noon sector the boundary is at 78.75° for Kp = 0o and moves equatorward by about 1.3° per unit increase of Kp. The boundary displays a slight asymmetry about the geomagnetic noon meridian. These dependences of the equatorward boundary of the Fliz may reflect similar dependences of the equatorward boundary of the polar cusp.