Multipoint observations of spatial and temporal characteristics of Pc 4-5 pulsations in the dayside magnetosphere and particle signatures.

2020 ◽  
Author(s):  
Galina Korotova ◽  
David Sibeck ◽  
Mark Engebretson
Keyword(s):  
Magnetic Field ◽  
Second Harmonic ◽  
Spectral Characteristics ◽  
Radial Distance ◽  
Recovery Phase ◽  
Strong Magnetic Storm ◽  
Long Duration ◽  
Field Lines ◽  
The Times ◽  
Generation Mechanisms

<p>We use  multipoint magnetic field, plasma  and particle observations to study the spatial, temporal  and spectral characteristics of Pc 4-5 pulsations   observed  in the recovery phase of a strong magnetic storm on January 1, 2016.   The magnetosphere was compressed and periodic increases of the total magnetic field strength occurred every 20-40 min at the times of generation of the pulsations.  The frequencies of the Pc4 pulsations varied  from 14 mHz to 25 mHz with radial distance. An explanation for this behavior can be given in terms of standing Alfvén waves along resonant field lines.  By contrast, Fourier analysis of the magnetic field observations  shows that the compressional  Pc5 pulsations  exhibited  similar spectra at different radial distances.  The long duration of the Pc5 pulsations and their nearly constant frequencies indicate that the plasma conditions in the morning sector of magnetosphere were stable for more than two hours.  The Pc4 and Pc5   pulsations displayed wave properties consistent with the second harmonic waves. The energetic particles   observed by Van Allen Probes and GOES 15  exhibited  a regular periodicity over a  broad range of energies from tens of eV to 2 MeV  with periods  corresponding to  those of the compressional component   of the  ULF magnetic field.   We searched for possible solar wind triggers and discussed generation mechanisms for the compressional Pc5 pulsations  in terms of drift mirror instability and  drift bounce resonance. </p>

scholarly journals Multipoint observations of compressional Pc5 pulsations in the dayside magnetosphere and corresponding particle signatures

2020 ◽  
Vol 38 (6) ◽  
pp. 1267-1281
Author(s):  
Galina Korotova ◽  
David Sibeck ◽  
Mark Engebretson ◽  
Michael Balikhin ◽  
Scott Thaller ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Phase Shift ◽  
Spectral Characteristics ◽  
Energetic Electron ◽  
Radial Distance ◽  
Frequency Doubling ◽  
Recovery Phase ◽  
Field Lines ◽  
G 13 ◽  
The Magnetic Field

Abstract. We use Van Allen Probes (Radiation Belt Storm Probes A and B, henceforth RBSP-A and RBSP-B) and GOES-13 and GOES-15 (henceforth G-13 and G-15) multipoint magnetic field, electric field, plasma, and energetic particle observations to study the spatial, temporal, and spectral characteristics of compressional Pc5 pulsations observed during the recovery phase of a strong geomagnetic storm on 1 January 2016. From ∼ 19:00 to 23:02 UT, successive magnetospheric compressions enhanced the peak-to-peak amplitudes of Pc5 waves with 4.5–6.0 mHz frequencies from 0–2 to 10–15 nT at both RBSP-A and RBSP-B, particularly in the prenoon magnetosphere. Poloidal Pc4 pulsations with frequencies of ∼ 22–29 mHz were present in the radial Bx component. The frequencies of these Pc4 pulsations diminished with increasing radial distance, as expected for resonant Alfvén waves standing along field lines. The GOES spacecraft observed Pc5 pulsations with similar frequencies to those seen by the RBSP but Pc4 pulsations with lower frequencies. Both RBSP-A and RBSP-B observed frequency doubling in the compressional component of the magnetic field during the Pc5 waves, indicating a meridional sloshing of the equatorial node over a combined range in ZSM from 0.25 to −0.08 Re, suggesting that the amplitude of this meridional oscillation was ∼ 0.16 Re about an equatorial node whose mean position was near ZSM=∼0.08 Re. RBSP-A and RBSP-B HOPE (Helium Oxygen Proton Electron) and MagEIS (Magnetic Electron Ion Spectrometer) observations provide the first evidence for a corresponding frequency doubling in the plasma density and the flux of energetic electrons, respectively. Energetic electron fluxes oscillated out of phase with the magnetic field strength with no phase shift at any energy. In the absence of any significant solar wind trigger or phase shift with energy, we interpret the compressional Pc5 pulsations in terms of the mirror-mode instability.

scholarly journals Multipoint Observations of Compressional Pc5 Pulsations in the Dayside Magnetosphere and Corresponding Particle Signatures

2020 ◽  
Author(s):  
Galina Korotova ◽  
David Sibeck ◽  
Mark Engebretson ◽  
Michael Balikhin ◽  
Scott Thaller ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Phase Shift ◽  
Spectral Characteristics ◽  
Energetic Electron ◽  
Radial Distance ◽  
Frequency Doubling ◽  
Recovery Phase ◽  
Van Allen Probes ◽  
Field Lines ◽  
The Magnetic Field

Abstract. We use Van Allen Probes and GOES-13 and -15 multipoint magnetic field, electric field, plasma, and energetic particle observations to study the spatial, temporal, and spectral characteristics of compressional Pc5 pulsations observed during the recovery phase of a strong geomagnetic storm on January 1, 2016. From ~ 19:00 UT to 23:02 UT, successive magnetospheric compressions enhanced the peak-to-peak amplitudes of Pc5 waves with 4.5–6.0 mHz frequencies from 0–2 to 10–15 nT at both Van Allen Probes A and B, particularly in the prenoon magnetosphere. Poloidal Pc4 pulsations with frequencies of ~ 22–29 mHz were present in the radial Bx component. The frequencies of these Pc4 pulsations diminished with increasing radial distance, as expected for resonant Alfvén waves standing along field lines. The GOES spacecraft observed Pc5 pulsations with similar frequencies to those seen by the Van Allen Probes, but Pc4 pulsations with lower frequencies. Both Van Allen Probes A and B observed frequency doubling in the compressional component of the magnetic field during the Pc5 waves, indicating a meridional sloshing of the equatorial node over a combined range in ZSM from 0.25 to −0.08 Re, suggesting that the amplitude of this meridional oscillation was ~ 0.16 Re about an equatorial node whose mean position was near ZSM = ~ 0.08 Re. Van Allen Probes A and B HOPE and MagEIS observations provide the first evidence for a corresponding frequency doubling in the plasma density and the flux of energetic electron, respectively. Energetic electron fluxes oscillated out of phase with the magnetic field strength with no phase shift at any energy. In the absence of any solar wind trigger or phase shift with energy, we interpret the compressional Pc5 pulsations in terms of the mirror mode instability.

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 A case study of dayside reconnection under extremely low solar wind density conditions

2008 ◽  
Vol 26 (11) ◽  
pp. 3571-3583
Author(s):  
R. Maggiolo ◽  
J. A. Sauvaud ◽  
I. Dandouras ◽  
E. Luceck ◽  
H. Rème
Keyword(s):  
Magnetic Field ◽  
Solar Wind ◽  
Magnetic Reconnection ◽  
High Speed ◽  
Radial Distance ◽  
Plasma Jets ◽  
Solar Wind Density ◽  
Ion Distribution ◽  
Dayside Magnetopause ◽  
Field Lines

Abstract. From 15 February 2004, 20:00 UT to 18 February 2004, 01:00 UT, the solar wind density dropped to extremely low values (about 0.35 cm−3). On 17 February, between 17:45 UT and 18:10 UT, the CLUSTER spacecraft cross the dayside magnetopause several times at a large radial distance of about 16 RE. During each of these crossings, the spacecraft detect high speed plasma jets in the dayside magnetopause and boundary layer. These observations are made during a period of southward and dawnward Interplanetary Magnetic Field (IMF). The magnetic shear across the local magnetopause is ~90° and the magnetosheath beta is very low (~0.15). We evidence the presence of a magnetic field of a few nT along the magnetopause normal. We also show that the plasma jets, accelerated up to 600 km/s, satisfy the tangential stress balance. These findings strongly suggest that the accelerated jets are due to magnetic reconnection between interplanetary and terrestrial magnetic field lines northward of the satellites. This is confirmed by the analysis of the ion distribution function that exhibits the presence of D shaped distributions and of a reflected ion population as predicted by theory. A quantitative analysis of the reflected ion population reveals that the reconnection process lasts about 30 min in a reconnection site located at a very large distance of several tens RE from the Cluster spacecraft. We also estimate the magnetopause motion and thickness during this event. This paper gives the first experimental study of magnetic reconnection during such rare periods of very low solar wind density. The results are discussed in the frame of magnetospheric response to extremely low solar wind density conditions.

scholarly journals DISTURBED MAGNETOSPHERE ON NOVEMBER 7–8, 2004AND VARIATIONS OF COSMIC RAY CUTOFF RIGIDITY: LATITUDE EFFECTS

2020 ◽  
Vol 6 (3) ◽  
pp. 40-47
Author(s):  
Olga Danilova ◽  
Natalia Ptitsyna ◽  
Marta Tyasto ◽  
Valeriy Sdobnov
Keyword(s):  
Magnetic Field ◽  
Interplanetary Magnetic Field ◽  
Neutron Monitor ◽  
Cosmic Ray ◽  
Recovery Phase ◽  
Survey Method ◽  
Cutoff Rigidity ◽  
Strong Magnetic Storm ◽  
Current Systems ◽  
Activity Indices

We have studied the latitude behavior of cosmic ray cutoff rigidity and their sensitivity to Bz and By components of the interplanetary magnetic field and geomagnetic activity indices Dst and Kp for different phases of the November 7–8, 2004 strong magnetic storm. Cutoff rigidities have been calculated using two methods: the spectrographic global survey method in which the cutoff rigidity is determined from observational data, acquired by the neutron monitor network, and the method in which particle trajectories are calculated numerically in a model magnetic field of the magnetosphere. We have found that the sensitivity of observed cutoff rigidities to Dst changes with latitude (threshold rigidity of stations) is in antiphase with changes in the sensitivity to By. During the recovery phase of the storm, the Dst correlation with By is significantly greater than that with Bz, and the Kp correlation with Bz is greater than that with By. The By component is shown to be a predominant driver of the current systems that determine the Dst evolution during the recovery phase.

Juno’s Exploration of Jupiter’s Magnetic Field and Magnetosphere

2020 ◽  
Author(s):  
Jack Connerney ◽  
Ron Oliverson ◽  
Stavros Kotsiaros ◽  
Dan Gershman ◽  
Yasmina Martos ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Attitude Determination ◽  
Radial Distance ◽  
Field Investigation ◽  
Opposite Polarity ◽  
Magnetic Star ◽  
Fluxgate Magnetometer ◽  
Solar Arrays ◽  
Field Geometry ◽  
Field Lines

<p>The Juno spacecraft was inserted into polar orbit about Jupiter on July 4<sup>th</sup>, 2016, performing close passes (to ~1.05 Rj radial distance at periJove) every 53 days. By the end of its prime mission, Juno will have circled the planet 34 times, uniformly sampling longitudes separated by less than 11<img src="data:image/jpeg;base64,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" width="4" height="13"> at the equator. The Juno magnetic field investigation is equipped with two magnetometer sensor suites, located at 10 and 12 m from the spacecraft body at the end of one of Juno’s three solar arrays. Each contains a vector fluxgate magnetometer (FGM) sensor and a pair of co-located non-magnetic star tracker camera heads that provide accurate attitude determination for the FGM sensors. A moredetailed view of Jupiter’s planetary dynamo is emerging as Juno acquires more periJove passes, providing spatial resolution beyond that already evident in the preliminary model (JRM09, a degree 10 spherical harmonic) derived from Juno’s first 9 periJoves. A complex and very non-dipolar magnetic field dominates the northern hemisphere, while a mostly dipolar magnetic field is observed south of the equator, where the enigmatic “Great Blue Spot” resides within an equatorial band of opposite polarity. The Jovian magnetodisc, formed by a washer-shaped disc of azimuthal (“ring”) currents, stretches magnetic field lines outward along the magnetic equator. With 26 equally spaced longitudes now available we can begin to address magnetodisc variability, finding a more or less stable system of azimuthal ring currents (few % variability) and a more variable (~50%) system of radial currents that supply torque to outflowing plasma. A new magnetodisc model greatly improves knowledge of the field geometry, independently verified via observations of the particle absorption signatures of Galilean satellites. A more systematic mapping of Birkeland currents above the polar aurorae also emerges from multiple passes. These and other developments will be presented with Juno now about ¾ of the way towards completion of its primary mission.</p>

scholarly journals Magnetotail response during a strong substorm as observed by GEOTAIL in the distant tail

1998 ◽  
Vol 16 (5) ◽  
pp. 528-541 ◽  
Author(s):  
A. Belehaki ◽  
R. W. McEntire ◽  
S. Kokubun ◽  
T. Yamamoto
Keyword(s):  
Magnetic Field ◽  
Magnetic Structure ◽  
Energetic Particle ◽  
Recovery Phase ◽  
External Variable ◽  
Flux Tubes ◽  
Magnetic Structures ◽  
Magnetospheric Configuration And Dynamics ◽  
Field Lines ◽  
High Beta

Abstract. Simultaneous energetic particle and magnetic field observations from the GEOTAIL spacecraft in the distant tail (XGSM≈ –150 Re) have been analysed to study the response of the Earth's magnetotail during a strong substorm (AE ≤ 680 nT). At geosynchronous altitude, LANL spacecraft recorded three electron injections between 0030 UT and 0130 UT, which correspond to onsets observed on the ground at Kiruna Ground Observatory. The Earth's magnetotail responded to this substorm with the ejection of five plasmoids, whose size decreases from one plasmoid to the next. Since the type of magnetic structure detected by a spacecraft residing the lobes, depends on the Z extent of the structure passing underneath the spacecraft, GEOTAIL is first engulfed by a plasmoid structure; six minutes later it detects a boundary layer plasmoid (BLP) and finally at the recovery phase of the substorm GEOTAIL observes three travelling compression regions (TCRs). The time-of-flight (TOF) speed of these magnetic structures was estimated to range between 510 km/s and 620 km/s. The length of these individual plasmoids was calculated to be between 28 Re and 56 Re. The principal axis analysis performed on the magnetic field during the TCR encountered, has confirmed that GEOTAIL observed a 2-D perturbation in the X-Z plane due to the passage of a plasmoid underneath. The first large plasmoid that engulfed GEOTAIL was much more complicated in nature probably due to the external, variable draped field lines associated with high beta plasma sheet and the PSBL flux tubes surrounding the plasmoid. From the analysis of the energetic particle angular distribution, evidence was found that ions were accelerated from the distant X-line at the onset of the burst associated with the first magnetic structure. Key words. Magnetospheric physics (magnetospheric configuration and dynamics; magnetotail).

scholarly journals Global Pc5 pulsations during strong magnetic storms: excitation mechanisms and equatorward expansion

2014 ◽  
Vol 32 (4) ◽  
pp. 319-331 ◽  
Author(s):  
J. Marin ◽  
V. Pilipenko ◽  
O. Kozyreva ◽  
M. Stepanova ◽  
M. Engebretson ◽  
...  
Keyword(s):  
Spectral Characteristics ◽  
Low Frequency ◽  
Recovery Phase ◽  
Magnetic Storms ◽  
Solar Wind Density ◽  
Helmholtz Instability ◽  
Strong Magnetic Storm ◽  
Transmission Mechanisms ◽  
Excitation Mechanisms ◽  
Using Data

Abstract. The dynamics of global Pc5 waves during the magnetic storms on 29–31 October 2003 are considered using data from the trans-American and trans-Scandinavian networks of magnetometers in the morning and post-noon magnetic local time (MLT) sectors. We study the latitudinal distribution of Pc5 wave spectral characteristics to determine how deep into the magnetosphere these Pc5 waves can extend at different flanks of the magnetosphere. The wave energy transmission mechanisms are different during 29–30 October and 31 October wave events. Further, we examine whether the self-excited Kelvin–Helmholtz instability is sufficient as an excitation mechanism for the global Pc5 waves. We suggest that on 31 October a magnetospheric magnetohydrodynamic (MHD) waveguide was excited, and the rigid regime of its excitation was triggered by enhancements of the solar wind density. The described features of Pc5 wave activity during recovery phase of strong magnetic storm are to be taken into account during the modeling of the relativistic electron energization by ultra-low-frequency (ULF) waves.

The Viscous Magnetosphere

1996 ◽  
Author(s):  
Charles F. Kennel
Keyword(s):  
Magnetic Field ◽  
Boundary Layer ◽  
Radial Distance ◽  
Velocity Shear ◽  
Low Latitude ◽  
Viscous Interaction ◽  
Field Lines ◽  
Layer Flow ◽  
Solar Wind Origin ◽  
Low Latitude Boundary Layer

This chapter describes how the magnetosphere is shaped by the tangential shear stress exerted at the magnetopause by collisionless viscosity. In Section 4.2, we discuss the low-latitude boundary layer (LLBL), which contains plasma of solar wind origin that has been transported across the magnetopause current layer. The velocity shear in the LLBL drives field-aligned currents into the ionosphere on the morning side and out of the ionosphere on the evening side (Section 4.3). These currents are of the appropriate sense to drive two-cell convection in the highlatitude ionosphere. The footprint of the LLBL in the ionosphere to which the field aligned currents connect is clearly identifiable by its characteristic particle precipitation (Section 4.4). The shear in the LLBL also generates 1-20 mHz PC 4- 5 micropulsations whose polarizations, tailward propagation, and phase speeds are consistent with the Kelvin-Helmholtz (K-H) instability (Section 4.5). The K-H vortices may couple to “vortex auroras” in the local afternoon sector of the auroral oval (Section 4.6). Vortex auroral dissipation may be responsible for a morningevening asymmetry in the viscous interaction and its manifestations. Organized vortical flows have been observed not only next to the magnetopause, but also near the center of the plasma sheet, accompanied by local quasiperiodic magnetic field oscillations and PC 5 micropulsations on the ground (Section 4.7). In Section 4.8, we discuss observations of a thick boundary layer flow on closed field lines next to the magnetopause 220 RE downstream. This puts us in a position to estimate the rates of particle and energy injection into the magnetosphere due to the viscous interaction (Section 4.9). Spacecraft crossings of the magnetopause last from a few seconds to a few minutes and are characterized by a rapid, distinct rotation of the magnetic field and striking changes in plasma density, pressure, flow velocity, composition, and energetic particle distribution (Williams, 1979a; 1980; Williams et al., 1979). A broader boundary layer lies just inside the magnetopause. The so-called low-latitude boundary layer was first identified at 18 RE radial distance in the magnetotail using Vela 4B (Hones et al., 1972) and Vela 5 and 6 (Akasofu et al., 1973b) low-energy plasma measurements.

Long duration Pc 5 compressional pulsations inside the Earth's magnetotail lobes

1995 ◽  
Vol 13 (9) ◽  
pp. 926-937 ◽  
Author(s):  
D. V. Sarafopoulos
Keyword(s):  
Magnetic Field ◽  
Travelling Waves ◽  
Mean Amplitude ◽  
Density Fluctuations ◽  
The North ◽  
Long Duration ◽  
Compression Region ◽  
Second Mode ◽  
Field Lines ◽  
Current Polarity

Abstract. Pc 5-type magnetic field pulsations are detected by the IMP-8 spacecraft well inside the Earth's magnetotail lobes. The three studied events with an average duration of 3 h and mean amplitude of ΔB/B=6.6% show a strong longitudinal oscillation. The clockwise polarization sense of the magnetic field arrowheads in the north lobe (as well as the counterclockwise in the south lobe) on the XZ plane is consistent with that expected when periodic solar wind lateral pressures squeeze the magnetotail axisymmetrically while moving tailward. In the two case studies, the latter property has been found to concur with quasi-periodic upstream density fluctuations detected by ISEE-3 and/or ISSE-1. The lobe magnetic field oscillations are classified in two distinct modes. The manifestations of the first mode are tailward-travelling waves detectable along the By and Bz magnetic field traces (i.e., with regard to the Bz the spacecraft encounters constantly the same conspicuous signature of south-then-north tilting of field lines around each local compression region). The second mode is associated with prolonged periods of extremely low geomagnetic activity and exhibits a signature along the By component inconsistent with travelling waves. Thus, the maxima of compressions occur simultaneously with the maxima of By excursions: a feature that is explained in terms of tail-aligned current density flowing at the boundary which separates the stable magnetic field in the tail lobe from the very irregular in the magnetosheath. In this case, the spacecraft was located in the vicinity of the high-latitude tail boundary and the observed By excursions are consistent with those anticipated by the tail-aligned current polarity, which is determined by the dominant By-component of the interplanetary magnetic field (IMF). On the plane YZ we observe an almost linear and circular polarization sense of the vector magnetic field for the first and second mode, respectively.

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